In my opinion going one size up (unless your child is thinner) wouldn't be a bad idea. My daughter is 4 years old and wears most of her dresses in a size 4-5 and I will order a size 6 next time. I originally ordered 5t and they were too small. The quality seemed good. Keep in mind they are NOT SUPER stretchy.
Lilax Girls Basic Length Leggings Girls' Clothing in Leggings
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Slaixiu Leggings Fleece Printing GP777 3P B 130 Girls' Clothing and Leggings
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Girls' Clothing and Leggings - Govc Winter Fleece Velvet Leggings
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Spotted Zebra Girl's Disney Star Wars Marvel Frozen Princess Leggings: Unleash the Fantasy!
46 Review
Thermajane Girl's Cozy and Comfy Thermal Underwear Long Johns Set with Fleece Lining
46 Review
Pack Athletic Performance Basketball Drawstring Girls' Clothing
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Dancina Toddler Leotard Flutter Sleeve Girls' Clothing for Active
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Amazon Essentials Little Short Sleeve Leotard Girls' Clothing for Active
61 Review