Kids are excitable, and sometimes when they get excited, they seem to lose control of their limbs. It's like something takes over and suddenly their arms and legs (and surprisingly heavy heads) are flying all over the place.
If you're unlucky enough to get caught in the crossfire and end up getting smacked, rammed, head-butted, or kicked, you discover just how much force kids are able to muster when something's got them all riled up.