My son loves this t-shirt for sports. It was quite thin/comfortable: my son is average height, a slim 6 year old and I bought him an XS and I could of course go up a size. The length is good for a size 7 but unless you are looking for something form fitting I would go up a size. According to UA website, XS is size 7, S is size 8, M is size 10-12, L is size 14-16 and XL is size 18-20.
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