You did what anyone else with a conscience would have done - I've done the same myself in the past. I parked the car in Sainsbury's car park and noticed that there is a small child (about 3) and a small dog in the adjacent car, unsupervised. I hung about a bit, but no sign of an adult. meanwhile the child is openig and closing the windows - so clealry the car is powered-up.
I took down the reg no, ran into the store and asked the stor staff to put out an announcement. They refused. I then asked them to call the police. Again they refused, so I got my mobile out and called themmyself. I stood with the car until the police arrived - a good 10 mins. At that point, the mother arrived with a full trolley and 2 other, slightly older kids in tow.
I told the police what I'd seen, and the mother's response was "Well, he wasn't supposed to be doing that"
. At that point, I decided to leave them to it in case she got nasty, but I could hear the Police giving her a rollocking.
If I hadn't spotted the child and taken action, what might have happened? At best, he might have let the handbrake off and rolled the car into the one in front. At worst, someone might have taken advantage of the situation and stolen the car with the child in it (this was Tottenham, after all).