My daughter is very slim and although the tights fit her legs well, part of the waist falls down. She is 6 years old and a petite size 6. She is very skinny and when she does gymnastics I see her slouching under her bathing suit.
Pros
They fit perfectly for my daughter who really doesn't like tights with legs. No more complaints!
Cons
The material may not "breathe" causing discomfort or sweating.