You need to let them know in no uncertain terms that actions like that will not be tolerated, at all. Go talk to them when you see them. Let them know that if it happens again the police will be called. Let them know why you will not tolerate this behavior. If they do anything but sit and stare, or apologize, Knock on the door and talk to the parents. Any grief from them, call the police. The police will probably give you the 'we didn't see it, so we can't(aka won't) do anything' line, but just having them show up shows how serious you are. This is a deadly game they played, and you had everything to lose, and didn't ask to be included.
You handled it much better than I have. This kind of things trips my trigger and turns me into a screaming lunatic.