A best bet are these Joe Boxer Toddler Boys' Blowfish Graphic Swim Shorts in Red (pictured) for $9. Choose in-store pickup to dodge the $5.55 shipping fee. That's $9 off and the lowest price we could find. They come in sizes 2T to 4T.
Update: It now drops to $8.10 when you add it to your cart.
Update #2: The price no longer drops in-cart.