File a formal complaint with the Post Office. They have the form online or at the KW PO. Even though you purchase the mailbox, it is the property of the US Postal Service, and mailbox vandalism is a federal crime. If or when they are caught, they face more than a slap on the wrist from a local JP. If they did it to your box, you can bet they have done it to others and will do it again, and may graduate to other, more violent forms of vandalism. The threat of being turned over to the feds can make a really good impression on the kids and their parents.