Both of my kids needed 100% cotton t-shirts for end of the school year tie-dye shirts. I don't personally like using undershirts as an everyday shirt and 100% cotton isn't as easy to find as it used to be. I bought them a 10/12 (they both wear a 7/8)  bcuz the reviews said the shirts ran small & I'm glad I did. They did shrink a little but I anticipated that. They're still a little big that they'll both get to wear them for a while before they grow.