Some kid got angry and sat on my head underwater. He was several years older and much bigger (I was 7 he was maybe 11 or 12). He was mad because I confronted him about stealing my toys (little miniature transformers not expensive but theft is theft. He had been accusedof theft by others but my Mom thought he was just being bullied, he had a cleft lip, and I should try to be his friend). Both our families went to the lake for the weekend and he was playing with one in the water and for some reason either didn't think I'd see him playing with it or wanted me to be mad. I said I was going to tell on him and he grabbed me and shoved my face into the sand in two foot deep water and sat on my head.
Luckily there was a bystander who stopped it, but that fucker was totally prepared to murder me over some plastic. I later found out he had done similarly violent stuff to other kids after I stopped being around him.
I don't know, I moved away not long after and don't talk to anyone from that town. I can't even remember his name, so I wouldn't even be able to look him up on Facebook
Kids can definately notice when another kid is off kilt. I think adults can see it often but often they need to stay somewhat neutral so that someone doesn't get stigmatized and ultimately has no chance of being anything but what people think.
In some ways we have made this worse by discouraging any type of conflict in children. In some ways they know how to dish out informal punishment possibly correcting bad character faults before people reach adulthood.