I would definitely look into the legal aspects.
If you have a fenced up dog in your back yard and some asshat kid jumped the fence and the dog eats him, that is not your problem. The kid should have not been on your property. Hell, if I went in my back yard and some kids were back there, a few rounds in the air from a 12 gauge would never have them come back.
All else fails, have someone sit in the back and just destroy these kids. My friends parents house had a bunch of random kids who would always come into their yard and hang out in the back by the trees and leave beer cans and break shit in all the lawns almost every weekend. One night we just staked it out and went after them with and old paintball gun with frozen paintballs and havent seen them since.