Do they stand on the fence to get onto the tree? If yes, maybe take that fence down? Or turn it into an electrified fence, ha ha! If they start climbing the tree from the ground, perhaps plant dense flowers all around the base of the tree trunk, extending outwards for a radius of 3 feet in all directions, so that they'd essentially have to crush all your flowers with their feet in order to get near the tree-- I'm sure they wouldn't dare do that!!! Is there a fence all around your property? If there is, you could keep a padlock on the fence gate, and place a "No trespassing" sign on the fence, or even right on the tree trunk. If you don't have a fence around your property, you might even consider getting one, although that is very expensive, I know. Do these kids' parents know what the kids are up to? Could you have a woman-to-woman, or man-to-man, chat with them and ask them to tell their kids to stop it? ...I love the attack dog idea!