I purchased this for my daughter who just turned 4 years old last month, I ordered the 3-4y size and it fit her big.
The butt area was WAY too large and you could see all her private parts and she's on the taller side of a 4 year old at 43 pounds and 39" tall. I had to sew the crotch area to make it shorter so her privates would be protected. The material is also very flimsy and is already pilling after just one week of use at swim class; although we go daily, my son has a 2pc I bought at Costco last year that has seen excessive wear and is in better shape than this suit.