My daughter is 12 years old. She wears 12 or 14 depending on the item. Unfortunately, she couldn't put them on her hips. Very high waist and not very stretchy. I'd say order (at least) one size up whatever you think you need.
Girls' Clothing and Leggings - Govc Winter Fleece Velvet Leggings
49 Review
ZukoCert Fleece Leggings Cashmere 2P B 130 Girls' Clothing for Leggings
61 Review
IRELIA Girls Leggings Modal Spring Girls' Clothing for Leggings
52 Review
Slaixiu Leggings Fleece Printing GP777 3P B 130 Girls' Clothing and Leggings
54 Review