This shirt was part of the uniform I needed to purchase for my two boys' dance classes. It is thick enough that they don't need an undershirt, soft, and stretchy, but it runs VERY small. I ordered size 7-8 shirts for my guys. It was able to get around my 4 year old's chest and tummy but it showed his belly button when he raised his arms. It couldn't even stretch around my 5 year old's chest. I realize the shirt should fit snugly, but it was short and tight in the armpits as well. I'm going to reorder a 9-10 for the guys. Both weigh about 45 pounds and they are 42 and 45 inches tall, if that helps anyone else for sizing.
This shirt is perfect for my 8 year old son. It solves the baggy , boxy look of wearing an undershirt. It is definitely form fitting. My child usually wears a size 8 and the 9/10 was exactly right. Our only issue is that there is a tag sewn into the neck that cannot be removed without damaging the shirt.