My response:
A 7 year old cannot appreciate his actions, or actions that are negligent. In fact, a court would never find that a 7 year old is negligent. For purposes of negligence liability, minors are not held to the same standard of conduct as adults. Rather, they are required to exercise only that degree of care ordinarily exercised by minors of like maturity, intelligence and capacity under similar circumstances.
Except in special circumstances, of which yours is not one, parents are not responsible for the negligent acts of their minor children. Parents are only responsible for the malicious acts of their children and, under the law, a 7 year old is incapable of forming malicious intent.
Fix the wheel.
IAAL