When I was about six years old my five year old brother knocked my two front teeth out when we were boxing.
HA! That just reminded me of a time when I was about 5 years old...my older sis and I were riding our trikes on the driveway. Anyway, she did something to piss me off, can't remember what, but when she got off her trike I floored it and rammed the front wheel of my trike into her heel. That metal part that covers the top of the wheel (whatever it is called), sliced a good way into her heel. She needed stitches, the poor thing.