Wow, those kids are idiots, but they do it because they've obviously done it before and gotten away with it.
My question to Diana, is what the %*&@ were an 11 year old and 14 year old doing with guns?!
I've never even seen a gun in my life, I don't think anyone should own one, unless you're a cop, and the minute you cause damage with said gun you sould be responsible.
Here, when your child causes damage to someone else's property, YOU are 100% responsible to pay the damages, doesn't matter who you are, if you're broke or rich, that's too damn bad. Teach your children not be be little buggers.
(The exception is accidental damage, but that has to be proven)
To the OP, I can understand that it might cause friction in the family, but it seems as though it already too late to avoid that. I would bill them for it, and stand your ground! You can't do much about the time you spend on your projects, but sending them a bill for the materials might make them think about disciplining their children. Who cares if they're angry... uh they have no right to be the ones who are angry, YOUR property got damaged, not theirs. They now only have bruised egos because they know everyone thinks they're a family of misfits.