My 7 year old son weighs 52# and is 50" tall.  He wears a size M in the Hanes Comfortsoft for boys which is a good size for him - still slightly big and roomy but doesn't look baggy on him.  I needed to buy a lot of t shirts for an art project and the Fruit of the Looms were cheaper by a dollar for a package.  I bought a large for him as I thought it would give him a little extra time to wear the shirt. After washing the shirts the L in the Fruit of the Loom is smaller than the M in the Hanes!!!  Plus the Fruit of the Loom is a lot thinner and rougher.  My daughter is also 7 years old and weighs 44# and is 44" tall.  The M in the Fruit of the Loom just fits her but is snug.

I should have spent the extra dollar for the Hanes even though it would have been a little pricier for the art project.  At least the kids would be able to wear their shirts for more than a month.  Very disappointed.