That's really disturbing, I didnt think you could build muscle at that age? There's a hormone imbalance that doesnt get sorted out untill puberty, I'm guessing steroids or something similar is involved.
That aside, it's actually not HEALTHY for kids to work out like that when there are young, their bones are still growing and they can seriously mess up their bodies for later on in life, excercising for kids is fine, but this sort of weight lifting should wait until they are older.