We got DD a two piece (tankini, but still showed significant tummy) when she was about 4.5. And it was PURELY practical. She frequently found that immediately after getting into the pool, she needed to pee. LOL And peeling the wet suit off her, and then trying to put it back on, was difficult and uncomfortable, and she needed help. So we got her this one because she can just pull down the bottoms and while still sticky, a lot easier than trying to shimmy a wet suit back up her body.
She still has one, though after her recent growth spurts, she likely needs a new one.