What would you do?

[keakar]

that's all you can do, call the cops and file a report so if your windows come up broken or find dents in the car you have documentation of what they were doing.

I would also install a nanny cam that can cover the car and the kids hitting balls at it in case it ever needs to go to court for you to collect damages for the repairs.

azzholes like them will not do anything and just laugh as their kids destroy your stuff but get the cops involved and that "usually" puts an end to it.

sadly when you have bad neighbors like that, putting up a fence is your only "real" solution so you don't have to worry all the time what they will do next.