I can’t see how you would be liable, even in America
Question
Were the keys in the car, was the car running, etc, what exactly happened, not that it should matter legally if the car was on your property, but if you leave a car running on the street or at a gas station the laws might vary
You should have partial insurance coverage, like i have on cars i dont drive, which is less than $10/month, in case of theft/fire, or maybe something in your homeowners policy
That is a rough break if you get stuck with a bill
Can’t imagine you would, especially if they caught the kid, what state are you in