The short answer
Pick one print or color block, buy it in two sizes, and judge each pair on its own fit. A dad and a four-year-old share almost nothing in proportion, so the print is the only part that should match. Hem position, waist, liner and what goes on top all get decided separately.
| Decision | Dad | Boy |
|---|---|---|
| Size by | Waist measurement, taking the larger size if between two | Height, and the size that fits now |
| Hem | Wherever he would wear any other shorts | Above the knee, so the legs can bend and climb |
| Waist cord | Personal preference | Short, tied and tucked. Taken out or tucked for slides and playgrounds |
| Liner | Comfort call | Check the listing, and try it on dry before the trip |
| Top | Solid rash top or light shirt | Solid rash top, plus a brimmed hat |
Check that both sizes exist in the print before you fall for it. Our family matching swimsuit listings sell each size separately, so a father-and-son pair is simply two items in one basket, and you never have to buy the rest of the family’s set.
Which prints still match at two very different sizes?
Color block and wide stripes hold up best. Large florals still work, but on a small pair they match by color more than by pattern.
The reason is how each design scales. On the color-block trunks in our range, the panels run in the same order from waistband to hem at both sizes, so a boy’s pair reads as a smaller copy of his father’s. An all-over floral behaves differently. Compare the man’s and boy’s photos on those listings and the flowers look about the same size on both, which means a dad’s trunks show a whole bouquet and a young boy’s show one and a half blooms.
| Design | How it travels between sizes | Good for |
|---|---|---|
| Color block with stripes | Reads as the same garment at any size | A dad who dislikes prints, and photos taken from a distance |
| Mid-size leaf or shell print | Pattern stays recognizable on a small pair | A clear match that still looks like ordinary vacation shorts |
| Large tropical floral | Cropped on a boy’s pair, so color carries the match | Boys old enough for a larger size, or anyone happy with a looser match |
| Narrow stripes across the whole garment | Consistent, but can shimmer on camera | Everyday wear more than planned photos |
That shimmer is moiré, and fine lines cause it most on whoever stands farthest from the lens. The explanation and fixes are in the three-color method for family photos.
How long should the trunks be on each of you?
Match where the hem sits on the leg, not the measurement in inches. A hem at mid-thigh on both of you looks like a pair, even though the two inseams may be many inches apart.
Dad’s length is about taste and height. If his usual shorts end in a particular spot, aim for that spot, and use the listing photos to see roughly where each style sits on the model.
The boy’s length is about movement. Pool ladders, sand and climbing all need a knee that bends freely, and a pair sized up for next summer works against that. Extra room holds water and drags, and the cord ends up doing the elastic’s job. As with other children’s swimwear, buy the size that fits this season.
Use the PatPat size chart for both of you, and check these range limits early, because they decide what is possible:
- Older boys. The boy’s half in the family sets we reviewed runs from 2Y to 8-9Y. A boy past that needs a shared color from boys’ swimsuits, with dad’s pair chosen to go with it.
- Babies and young toddlers. Those same sets usually start at 2Y. Under that, match by color and follow the baby swimwear guide, which covers swim diapers and coverage.
- Larger dads. Many men’s ranges stop at 2XL and a smaller number go further. Filter for his size first, then choose from what is left.
What should you check on a boy’s waistband and liner?
The elastic should hold the trunks up without help, and the liner should be something he will actually keep wearing. Both are easy to test at home before the trip.
Waistband. Most pairs we reviewed in toddler boy swimsuits and the family sets pairs an elastic waist with a tie cord. Put them on him with the cord untied and have him jump a few times. If they stay up, the cord is only fine-tuning, and you have the option of removing it on some days. If they slide, he may need the cord tied every time, or a smaller size.
Knots and bathroom trips. A wet double knot is a real obstacle for a newly potty-trained child who cannot wait. A single flat bow, with the ends tucked in, holds well enough and comes undone with one pull.
Liner. Listings vary. Some state a lining and its fabric, while others mention only the outer fabric, so read the fabric line rather than assuming. Some boys are fine with a built-in liner and others complain as soon as sand gets into it. A short wear around the house will tell you more than the description can.
Pale panels. Several color-block trunks have a white lower section. On those, confirm a lining is listed, then check them wet in the sink at home.
Swim diapers. A child not yet potty trained wears the swim diaper first and the trunks over it, so check the waist and leg openings with the diaper on.
When is a waist drawstring actually a risk?
The hazard the CPSC describes is a cord end catching on something while the child keeps moving. That makes it a matter of specific places, not a reason to skip tie-waist trunks, which are a common design in this category, ours included.
| Situation | What can happen | What to do |
|---|---|---|
| Beach or shallow pool play, with an adult watching | In our judgment, mainly a bow coming undone | Short bow, ends tucked inside the waistband |
| Water slides | A cord or knot could catch on the way down. This is our own caution, not a CPSC scenario | Take the cord out if you can, or tie it flat and tuck it. Check the park’s clothing rules |
| A playground on the way back from the pool | A knot catches in a gap at the top of the slide as the child descends | Remove or fully tuck the cord, or change into dry clothes first |
| Around a school bus or shuttle | A long trailing cord catches on the closed door of a moving vehicle | Same tuck. Trailing length is the problem |
| Near pool and spa drains | Swimwear can tangle in a drain cover | Keep children away from drains, and stay out of any pool with a broken, loose or missing cover |
The playground and bus scenarios come from the CPSC, whose drawstring guidance describes toggles or knots catching at the top of playground slides and a jacket’s waist cord caught on the closed door of a moving school bus. That guidance was written for children’s upper outerwear such as jackets, and it does not mention swim trunks, so applying it here is our own reasoning. For that outerwear it limits waist cords to 3 inches outside the channel, with no knots or toggles on the free ends. It sets no limit for trunks, but the figure is a useful yardstick. If the tied ends hang longer than about 3 inches, shorten the bow or tuck them.
The drain point is on the CPSC’s Pool Safely sheet, which says bathing suits and jewelry can become entangled in a drain cover. It looks uncommon among reported cases: the 2018 CPSC staff entrapment report counted no clothing or jewelry incidents from 2013 through 2017, with reporting for the later years still coming in. The same report lists 11 children hurt or killed through hair, limb or body entrapment, so the rule about staying away from drains holds whatever a child is wearing.
If the cord is not sewn in place and the elastic passes the jump test, pulling the cord out is the simple fix for a park day. What to wear to a water park covers rides, fastenings and which suit to bring.
Neck cords are a different matter. The CPSC treats hood and neck drawstrings on children’s upper outerwear in sizes 2T to 12 as a strangulation hazard, so any hooded cover-up or towel poncho you add for a child should have no cord at the neck.
Does trunk color change how easily you can spot a child in the water?
Possibly, but the evidence is thin: one company’s visual tests, known to us through press reports. It is not a safety measure on its own, and it should not decide which trunks you buy.
ALIVE Solutions, an aquatic safety company, tested 14 swimsuit colors in pools and a lake, as reported by the Deseret News:
- Light-bottomed pools. White and light blue virtually disappeared. Neon pink and neon orange did best, and darker colors showed up but could be mistaken for debris.
- Dark-bottomed pools. Neon yellow, green and orange were the most visible.
- Lake water. At 18 inches most colors disappeared, and neon yellow, green and orange did best. By two feet down, virtually all of them had gone.
- Patterns. Small patterns made little difference, while large dark patterns reduced visibility.
Treat those results with care. This is a company’s own test of how colors looked in the water, which we read through press coverage including TODAY. Nothing in that coverage measures how quickly anyone spots a real child, and TODAY describes the lake water as low visibility, so other water may differ. We could not reach the company’s own write-up and did not find a peer-reviewed version.
The lake finding is the one to remember: at two feet down, virtually every color had gone. The matching trunks we reviewed are mostly prints and non-neon colors, including blues, teals and white panels, which were not the top performers, and we have not tested any of them. A neon pair would not make a child safe in the water either. The CDC’s advice is a responsible adult supervising closely and constantly, without phones or other distractions, and it applies to children who have had swimming lessons too.
If color matters to you, there is an easy way to have both. Keep the matching trunks and put the boy in a bright solid rash top, which covers more of him than the trunks do. That is our reasoning from simple area, not a test result.
What goes on top so sun cover does not stop at the waistband?
A solid rash top on both of you. Trunks leave the whole upper body bare, and a solid top keeps the attention on the matching print below.
The AAP’s sun guidance is to dress children in cool, comfortable clothing that covers the body, choose a tight weave, add a hat with an all-around 3-inch brim, and limit sun exposure between 10 a.m. and 4 p.m. An ordinary tee is a weak substitute in the water: the Skin Cancer Foundation puts a white T-shirt at about UPF 7, and at only 3 once wet.
A few points on choosing the top:
- Pull the color from the trunks. Navy over navy-and-yellow stripes, deep teal over teal color block. Father and son in the same solid top is a match in itself, and it gets a shy dad into the photo with his shirt on.
- Buy tops separately if you need to. Most of the trunk listings we reviewed do not include a rash top for dad or boy, so plan on sourcing them on their own.
- Trust a rating only where it is stated. Any rating belongs to that specific garment and appears on its product page. What the number measures, and why it falls when fabric is wet or stretched, is in what UPF 50+ actually means.
- Babies are their own case. Under 6 months, keep them out of direct sun, relying on shade and clothing first as the AAP advises.
What if one of you does not want to match?
Drop to a shared color. Trunks in the same color family as his son’s still photograph as a pair, and swim trunks are an easy ask, since a dad can wear his on any other day without anyone knowing they match.
If dad balks at florals, try color block first. If a boy is old enough to object, let him pick between two prints you can both live with. The wider approach, including when boys tend to stop wanting to match, is in father and son matching outfits.
Will the pair still match next summer?
Probably not, so plan on replacing only half of it. Dad’s trunks may have years left, while the boy’s will likely be outgrown by next season, and the same print may no longer be sold by then.
- Order both pairs together. Stock can run out one size at a time, which makes a pair harder to complete later.
- Pick a pattern with an obvious main color. Next year the boy can go into any new pair in that color and still look coordinated with dad’s old ones.
- Treat both pairs the same. Rinse both in fresh water on the same day and follow each pair’s care label, since instructions differ between listings.
Why swimwear ages so quickly, how to store it, and how one print can run across the whole family are in matching family swimwear.
Frequently asked questions
Can I buy just the dad and son pieces from a family set? Yes. The family listings we reviewed sell each size separately, so you add one men’s size and one boy’s size and nothing else.
What age range do matching boys’ trunks cover? In the sets we reviewed, usually 2Y to 8-9Y. Outside that range, match by color using separate pairs.
Should I remove the drawstring from my son’s swim trunks? In our judgment, not for ordinary swimming if the cord is short and tied. Do take it out or tuck it fully for water slides and playgrounds, as long as the elastic holds the trunks up on its own.
Should I buy my son’s trunks a size up? No. Loose trunks hold water and drag, and the waist then depends on a tightly tied cord.
What color swim trunks are easiest to see in the water? In ALIVE Solutions’ company-run tests, neon colors did best and white and light blue did worst in light-bottomed pools. Close, constant supervision matters more than any color.
Do swim trunks need a liner? Not necessarily, and listings differ on whether one is included. Check the fabric line, and if a pair has white panels, make sure it is lined before relying on it wet.
What should a dad wear with matching trunks if he prefers to stay covered? A solid rash top or light shirt in a color from the print. The trunks carry the match, and he gets sun cover as well.
About this article
Written and fact-checked by the PatPat Editorial Team. How we research, source and review what we publish is set out on our editorial standards page.
Method. We reviewed the listings, photos and fabric details of swim trunks currently sold in our family and boys’ collections, and read the safety sources below. We did not wear-test any trunks or measure their UPF or visibility in water. The notes on print scale, hem position, liners, water slides and tucking cords are our own practical reasoning, not research findings, and the size ranges described may change as stock moves.
Sources. Drawstring hazards and limits on children’s upper outerwear: CPSC, Drawstrings in Children’s Upper Outerwear. Drains and swimwear entanglement: CPSC, Pool Safely Water Safety Steps and CPSC staff, Circulation Entrapment report, 2018. Swimsuit color tests by ALIVE Solutions, as reported by the Deseret News and TODAY. Supervision around water: CDC, Drowning Prevention. Sun protection through clothing: AAP, Sun Safety and Skin Cancer Foundation, Sun-Protective Clothing.
Last updated 5 November 2026 · Next review November 2027
