You ever play fight, Chasmfiend? Roll around in the dirt with a friend? It's all in good fun, right?
Besides, wouldn't you want them to be able to protect yourself in case the ever need it when you're not around?
Breaking bones is usually pretty obvious. It tends to hurt real bad and sometimes you can feel the break through the skin or in the way it moves.
If that happens, y'gotta keep it still and get 'em to a doctor... or here. That's better, these days, actually.
But I wouldn't worry too much! These kids of yours probably know where the line is. If not, that's what you're there for!