My daughter is 4 but a little bigger than most 4 year olds. We got her size 6 or 6x. It is a tight squeeze. She isn't overweight.

We had to change into it in a hot room and trying to pull it up on her ended up ripping it somewhere. We could hear it, maybe in the elastic. Now it doesn't fall right on her anymore. She's worn in twice and I'll need to get her a new bathing suit already.