I think the scenario in this thread would be an interesting one to take to the small claims court. Would the judge consider the parent negligent in giving a four year a trolley in a car park to take back to the trolley store. I don't think it is an unreasonable argument to make.
It would seem to be a logical case on that angle. Could the parent reasonably expect the 4 year old to safely (to himself, others and others property) return the trolley.
The evidence would seem to indicate no.
You can sue a parent for negligence in regards failing to supervise, suing the child would seem harsh, but if you did and won then you would be in line for all their birthday money, sweet money etc
The child would normally be covered if they reside in a residence in which public (private) liability insurance existed. This often isn't outside your property boundary though