Nothing they could do as I was technically the oldest (I say technically as I'm the middle, but my older sister is severely mentally challenged, so I've always been the most mentally mature one). I was also, allegedly, a very quiet baby according to my parents.
I did, however, accidentally break or fracture my little brother's fingers once. I was trying to close a cabinet and hadn't noticed he had his fingers in there. I kept trying to close it and he, like myself, was a quiet baby so he didn't cry out when I smashed his fingers. I was about three at the time. That said, what's the most physically damaging thing you've ever done to someone, intentionally or unintentionally?