Yeah I'd have to agree with Boxer. The kids need to learn. I would let them know that half of their allowance is going toward that damage! They'll soon learn to open the door slowly.
But the other idea is good too. Just try to park somewhere that the kids either can't hit a car next to you, or don't have the opportunity to. If they complain about parking far away, tell them that you wouldn't have to park there if they were more careful of other peoples' property around them.