I am not sure where the car was parked makes any difference on liability, OP has said his son caused the damage so in effect this makes the OP (as the father) liable. .
If it was parked on private land it could have a bearing, but from the little detail given I would assume it was on a regular road.
If the car was (partly) parked on the pavement leaving little room for a child to cycle through the gap surely the car owner would be (partially) liable.