I agree that there's absolutely no excuse for "forgetting" a child in a car like this. I have two of my own and can't imagine ever "forgetting" them anywhere. That's awful what happened to that kid

(WTF kind of name is zipporah anyway?)
Still, clearly it happens regularly all around the country and the world, and not just to people you'd assume are stupid (druggies, abusive parents etc). That suggests there are times when people might be tired or their brain otherwise simply zones out and they forget something major like that. Usually it doesn't end in death or major injury, but sometimes it does.
If we're talking about an actual parent and not an otherwise negligent idiot or a psychopath, the pain of losing their child in such a way is going to do far more than any punishment could ever do. Harsh punishment in that case does not accomplish anything: it will not bring the child back, it will not serve as a deterrent for future incidents, and it will not be any more severe than what the parent already experiences.