Well, there's no reason that a 12-year-old couldn't be alone in a parking lot to begin with, so I'm not sure why a 12-year-old couldn't be alone in a car (as long as they can get out... leaving somebody trapped in something is a bad idea whether it's a house or a car, a kid or an adult). Yes, somebody can break in to a car, but they can also break into a house, etc. That's assuming that the 12-year-old is of sound mind and has been taught how to recognize reasonable dangers (including, in a car, getting too hot) and knows what to do in emergencies, etc.
Collecting unattended kids at casinos and taking them to the casino daycare, then charging parents the hourly rate to get them back, seems a brilliant idea. Assuming they check the cars as well, etc. I'd at least do that for any kid who looks under a certain age, depending on the casino's/state's rules. If the child is old enough and the casino allows them to be unattended, maybe a system where the parent can check their child in upon arrival and get a sticker that shows that the child is old enough to be unattended in areas where kids are allowed, and all stickerless kids are collected and brought to the daycare.