I really hate my neighbors, part deux.

[Slartibartfast]

I think given the age there might be some confusion on the kids' part about "right" versus "wrong."  I am NOT condoning the behavior, but I don't think they were being malicious.  They probably didn't think it through and thought it was funny.

By age 8, most kids have a pretty good idea of what is "right" and what is "wrong."  Now, their impulse control may still be a little off, and they may not WANT to do the "right" thing, but they KNOW what it is.  It's even MORE important, in my opinion, for there to be repercussions for this - otherwise the little voice telling them the right thing to do could get shut out for good.

[Clara Bow]

I'm not a Grinch, but I would call the cops pronto. If those kids' parents aren't going to teach them to behave then they deserve to have the police knock on their door and give the boys a little talk about vandalism and other people's property.
Eight year olds know better than to take things that do not belong to them. My three year old knows better than that! I think if it happened again, I would call the police (main number, not 911) and tell them that I wanted to report a case of vandalism and have a car come around and talk to the kids reponsible. If nothing else that should keep them out of your yard.
Whatever you decide to do, talk to their mother. If she's as nice as you say, then she should speak to the kids directly....I hope...