My dd was doing this regularly about two months ago, she'll be three soon, she would just go up to other kids and whack them over the head no provocation, my initial instinct was to scoop her up, remove her from the situation, soon realised this was exactly what she wanted, attention, so then I tried comforting the 'hittee', that didn't really have any effect either, she seemed fascinated by the cause and effect, the crying and howling that she alone had triggered, then she started to look as though she felt sorry for the hurt she caused, I made a big fuss of the other kid, kept telling her 'look, you've made him cry', then a couple of times I threatened to leave the playground/party etc. gave her a warning said if you do that one more time then we're going..the threat did work and then she started stroking other childrens' heads, kissing them and hugging them...we do have the occasional lapse but all in all she's turning out to be a considerate child...but I know it's horrible, you think it's only your child but it's SO common, it does not mean that your child will have a long-term problem, you must definitely express your disapproval in a firm way, I think I was a bit wishy washy about that initially, then I started getting down on her eye level, making her look me in the eye, pinning her arms to her sides firmly, no shouting but just telling her that she had just hurt someone else, it's very frustrating and it did take her months to really properly stop but now I don't feel as though I have to hover over her at parties or playgrounds. Don't let other parents make you feel as though you're doing anything wrong. Ask the 'teachers' what methods they would recommend, ask them what their tactics are, communicate with these people as it essential you work together and consistently. Good luck