Before posting here obvious things like "don't leave children in a vehicle," please read this article: Fatal Distraction: Forgetting a Child in the Backseat of a Car Is a Horrifying Mistake. Is It a Crime? It is really good, it won a Pulitzer Prize for a reason.

I would say that you don’t leave any living being in the car, which is not capable to opening the door with ease by itself, when temperatures are elevated outside!!! This counts for elderly, immobile or physically or mentally disabled people as well as for any animal - dogs die often due to leaving them alone in the car when it’s hot, sometimes the window slightly open, though this does not help.

So, please, take action if you see anybody or any animal suffering in a car, because it is hot. In some countries you can call the police.