There are parents who neglect or abuse their children. It's fortunately rare, but it does mean that children also deserve protection from their parents, when necessary.

Leaving a child in a locked car is a bad idea, even when it's totally safe, and even with a window cracked. (In fact, leaving a car window open when you're not there is also a bad idea, though a few minutes probably won't hurt.) Honestly, in that situation, it's better to leave the car door open, so the child can get out when necessary. If you don't trust the child to make that decision, don't leave it alone.