We ordered the 6 month size for our son, who is 6 months and average height and slim (25th%). They were way too small. After just a few minutes he was unhappy and the elastic was digging in and leaving bad marks on his tummy. We got the 12 month instead and they fit well. So, I wanted to begin by suggesting you order a size up!

The shorts seem great. They stay in place well with good elastic and the diaper part is sturdy. We've had liquid leaks (no big deal) with some other cover-style cloth swim diapers, but these seem even to keep the liquid in because they're nice and padded. Fortunately we've only had wee at the beach and pool so far, so I can't report first hand how they deal with poop, but I feel very confident.