My daughter is 8 years old.
It's thin and long and after reading the reviews I bought her a small one. They fit perfectly and are very warm.
C9 Champion Girls Supersoft Bright Girls' Clothing and Active
46 Review
Amazon Essentials Little Short Sleeve Leotard Girls' Clothing for Active
61 Review
Dancina Toddler Leotard Flutter Sleeve Girls' Clothing for Active
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Hollhoff Gymnastics Playgrounds Shorts - Breathable Clothing for Girls
48 Review