Just to add to all the kids in cars stuff...
When I was 11 (OK, so not quite 12, but close enough) my mother used to lock me in the car when she went to pick up stuff. Our car had child locks on all the doors (stupid feature) that could only be opened from the inside with the remote control key thing. I was never left to die of heat stroke, although it could easily have happened. I never liked it.
Once a guy broke in while I was in there, and told me gruffly to get out. When he saw I couldn't, he made moves to drive off with me in the back of the car. Very fortunately, my mother was returning and had seen the guy force his way in, and started running. She managed to stop him in time. And then, the entire incident was blamed on me.
You should not leave a kid in a car unattended in the same way you shouldn't leave them anywhere else unattended. Apart from heat stroke (which few 12 year olds, let alone adults it seems, can recognise in themselves or younger siblings, let alone know the gravity of or properly deal with), there are emergency situations like the one I was forced into.
I have absolutely no sympathy for people who leave kids alone in cars and then try to find excuses for it. None. It doesn't matter what their excuse is; they have proven themselves incapable of caring for a child responsibly. Absolute scum. People can claim memory problems or a change in schedule or whatever they like--it doesn't alter the fact that, in perfectly normal circumstances which one could reasonably expect to occur again at some point, they put their child in danger. If they did it once, there is little stopping them from doing it again the next time a similar situation comes along. If it has to happen even once before they realise that it's dangerous, despite all the information out there, then that just isn't good enough. Do they not realise that fire is dangerous until they let their kid get burned? You don't get second chances with child safety. A person like this cannot care for a child.