Your best answer will be to call your local PD, tell them the situation, and ask what your rights are involving detaining the suspects.
As I am not a lawyer or a LEO, this is purely a guess, but I think you could be getting yourself into hot water since you are a) not in any physical danger and b) are threatening kids (you never said how old, it would be worse the younger they are) with deadly force over property damage. Here in NC the use of deadly force to defend property is a no-no. Aside from any legal implications, I would think in this "Nobody's responsible for their own actions, sue-happy society" we live in now, some ambulance chaser would also love to slap you with a civil suit for false imprisonment and of course, the pain and anguish you caused these model citizens. If you can, take a picture, get the best description you can, and call the cops. They may be able to nab the little SOB's while they are still riding their BMX bikes home.