Hi there and thank you for your question,
I will try to assist you with your legal question but please feel free to ask as many follow up questions in order to clarify your initial question. If you have a new question, you must please open a new thread.
Look, the legal question comes down to whether your son has the ability to understand if he's done something wrong and if he understands the consequences of his actions. At 6 years old, I don't think that this is the case.
Legally therefore, I don't think that the owner of the car will be able to hold your son liable for the damage. And please understand, the owner of the car can't hold YOU liable - since you never threw the stone.
I think that you must tell the owner of the car that your son didn't know what he was doing was wrong, and that he was only trying to protect himself from the other children, and that it was an accident. Your son didn't throw the stone at the car in an attempt to cause damage, it was an accident that the stone hit the car.
So, legally speaking, your son is not liable for the damage, and you're not liable for the costs.
Tell him that he must claim against his insurance for the repairs and that you will not be bullied into paying.
If he threatens to go to the police, tell him to go. Explain to the police what I've said above - that your son can't be held liable because he's so young. The police will tell the owner of the car to leave it alone and go away!
If my answer hasn't provided you with enough clarity regarding your initial question, please send me a REPLY with follow up questions so that I can continue to assist you in this same thread for as long as you need. Let me know if you need more advice - don't just rate my answer as "bad".
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Good luck and best regards,