You probably shouldn't put Rat Poison on the stones, incase the brats' die, then you would probably be held liable.
I despise these parents who always defend their children's bad behaviour, no wonder the kids behave like that. Do you have a separating wall and gate? Perhaps you could build the wall higher between the two properties and place a notice on the gate saying "Private Property"?
You could contact Social Services, but they sound like a pretty scummy family, who will probably suspect you and retaliate. However, it's a bit worrying these young thugs throw other kids in front of cars.