So lets deal with the actual events instead of what ifs.
Fact: The mom left her kid for who knows how long inside a locked car. Fact: The kid started choking on a piece of candy. Fact: The kid was only saved because of the friend.
Fact: If the mom had taken the kid inside with her like a responsible parent would, the kid might or might not have choked. If they did choke, the parent could intervene immediately.
Based on your rationale no parent should ever leave their child's side. Ever. Because if they do, something bad might happen. Little johnny might choke, he might get kidnapped, he might go into respiratory distress, he might poop himself and start eating it, etc. The possibilities are endless and we should never expose children to any sort of risk.
Newsflash - people just one generation ago left their kids in the car unattended while they did minor errands. And guess what? Most of those kids are now well adjusted adults. The fact that something might happen is not justification for a law prohibiting something that is in fact quite safe, particularly when a parent exercises reasonable judgment.
FWIW I am a parent of two small children. I have left both of them in a locked car while I do minor errands (e.g., taking money out of an ATM), particularly when the car is in plain view from wherever I am. If the car will be out of sight for any length of time, the kids go with me.
Also - I find it extremely annoying that your kind of people feel like it is somehow your job to tell everyone else what they must do, when in fact most people would exercise reasonable judgment to avoid exposing their kids to unreasonable risks of harm. Mind your own business. Your just like my idiot neighbor who stood at the end of my driveway to "make sure my son was ok" when he (my 5 year old son) was playing up by my garage some 200 feet from the road where I could easily watch him through the window (and I was watching him). And no, the neighbor is not a pedophile (See- its easy to predict where your worry wort mind will go). But I'm sure you would say that it should be criminal for kids to play in the front yard without a parent present, right?
Forgive me for being callus, but it is not the government's job to parent its citizens or the children of its citizens.