I love these shirts! My boys wear them to dance class. They're long enough to fully cover the waist of the leggings, and the sleeves don't impede movement or ride up. They are very thin but comfortable, ideal for physical activity. After several washes they have become even softer and shrinking has not affected their fit. I think they are meant to be worn as tank tops and for that purpose I think they would be very nice without being bulky under a top layer. My boys are almost 4 and we are size 3-4T. They are 38 inches tall and weigh just over 30 pounds. I think if your child is medium or plus size for their age I would order a size up.
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