ds (2 and a bit) whacks other children if they want a toy he is playing with. Every time he does it, he always (without fail) has to sit on his bottom away from the toys for a while. So at soft play type places or at the park I find I follow him around to ensure he either doesn't whack or to deal with him if I am a second too late.
Other children whack him and their parents don't seem to be nearby so they don't get in trouble. I sometimes wonder if I should just let him get on with the whacking and being whacked and chill out...