The hitting a child with a belt is a weird analogy. I don't think it's as acceptable as you think it was. Yes we all probably experienced it growing up, but how many times do you see a child given the belt in public? Did you ever see someone walk up to a parent with a crying child and ask 'why haven't you given them a flogging? You can use my belt if you want'. It's always done behind closed doors out view. An adult hitting child is always likely to spur negative emotions in people and opinions are likely to change.

That is completely different to kids in cars, there wont be a time where we will decide 'Hey maybe leaving them in the car is better.', it just wont happen.

Society views always change, that should be obvious to anyone. Events that shock people change the way we think about certain situations. Kids dying in cars as hot as oven's is one of them.