People need to get the message that at a minimum they are going to do significant jail time if they allow this to happen, no matter what the excuse. Except for the truly stupid and drink/drug-addled, this type of negligence would probably come to an end quickly once adults realize they will face serious repercussions.
Someone really needs to comes up with some sort of invention, that triggers an alarm if the baby is still in his/her car seat
For most parents, I couldn't imagine repercussions more serious than knowing you were responsible for the agonizing death of your child. I doubt added prison time will be much of a deterrent.