I do recommend if your child (boy) wears size American Medium go with XL. I child wears XL go with XXL and you will be happy with size. For the price you can't beat the instant gratification your child will have when he sees the shirt.
I am a returning Light in the box customer and I highly recommend their products. Be advise, some may take a bit getting use to how the products work when pertaining to sizes. When in doubt go 1 or 2 sizes higher.
Happy Lighting...Wayne's World Party on...Garth.