I feel your pain, had the same issue with my 318iS a few years ago when I was living in an estate. Talking to kids will only make them do it more, and sometimes even if you talk nicely to the parents they don't understand. I'd say best thing to do is set up a camera (doesn't have to be a hugely expensive one) and if they damage the car, fix it at BMW and send them the bill. Maybe they'll cop on then

Edit: Ofcourse they should pay for any damage! You surely have the right to NOT have your property vandalised!