The driver of the parked car did nothing wrong--the car was parked and not moving. You son hit it, which is careless or  negligent (either for your son, to hit a stationary vehicle; or for you, to not supervise your child while he was riding the bike, if he could not ride it safely); therefore, the car's owner is entitled to recover the cost to repair the car and has no obligation to take into account or pay for your son's injuries. It's the at-fault party which bears the liability and costs in cases like this.If you don't believe his cost to repair, you may offer to pay only what you think is appropriate or fair; if he does not accept that, he may try to sue you, at which time he can try to prove in court that the damages are what he says they are.