Like I said in my post if this person is prepared to drive off without calling parents/police then a real concern for Marple ??
Possibly. But you said they did stop to check that your son was alright, which he was.
How old is your son? It's not that unreasonable for her to have driven off once checking he was ok if he's, say, a 14-16 professing that he's fine (although I'd probably have stayed). Obviously a different story if he's only, say, 7 or 8.
Also, AIUI there's no need to report traffic collisions to the police if there are no injuries (although that may have change or may be different in the accident involves a pedestrian or minor).
Insurance is possibly a factor. In Marple (and the area of Marple in question) more likely that she didn't want to lose a no claims bonus than not having it (although that depends on whose fault the collision was).
If you do manage to track her down, what sort of resolution are you after?