Walmart cuts up to 75% off a selection of men's, women's, kids', and baby swimwear, with prices starting from $4.97$4.66. (Prices are as marked.) Shipping adds $4.97 or get free shipping on orders of $50 or more. Some sizes are limited. A couple of best bets:
Walmart offers the Disney Pixar All Stars Toddler Boy's Swim Trunks in Blue Stripe Print for $6. Opt for Site-to-Store pickup to dodge the $4.97 shipping charge. That's the lowest price we could find by $9. It's available in sizes 12 months to 5T.