He had the authority to tell them to get off his property. But constant empty threats? Give me a break. All that's going to do is egg the kids on and he should have known it. I think if he'd have shown the kids an appropriate amount of respect he might have gotten some back. Adults may have authority over kids, but I think that kinda relies on them actually acting like adults.
The kid in the background saying "He's just a kid! You're crazy!" was great, though. Good on him (the kid).