Five year olds

Since when has it been acceptable and normal for a child to make fun of his peer, both of them being five, because his peer happened to call a bruise or a cut, a "boo-boo?" I don't know, maybe I'm just sensitive since it is, my nephew, but still, they're five! I really hope that because of this, my nephew will get stronger and not become someone who likes to keep to himself in fear that the next word he'll say will just be as unacceptable as "boo-boo" is to his peers. I just want to hug him so badly now...