I used to work retail. I got to watch parents and small children a lot. I'm convinced that many modern parents, who were plopped in front of a TV at age 3, don't know much about kids or the world.

Let us take the case of a small child left in a car. Let's assume that the child is old enough to toddle. We don't just have to worry about a criminal abducting the child.

Case 1: The child gets afraid or bored, decides to go find parent. Child enters traffic, gets hit by a car.

Case 2: Child, C, decides to open car door. Person P is driving into the slot next to C's. P's car runs into C's door, doing $$ damage and injuring C and P.

Case 3: Location is a grocery store. Child gets out, walks toward door of store. Shopper, S, is walking out with armload of groceries. S activates the exit door, child walks into swinging door, catches steel door edge with middle of forehead. (I saved a little girl this way myself, once. Her mother did not appreciate my comment.)

Case 4: It is bitterly cold out. Below zero F. Child gets out of car, starts wandering around, is in danger of frostbite or exposure. (I know from the news and from friends who are parents that small children have no sense of danger or even discomfort from cold. They cannot step outside and tell themselves, "Damn, it's cold!")

Case 5: Katlaughing's story - car catches fire. Or starts rolling. Or (as mentioned) gets very hot. Or it's idling, and exhaust gets in. Cars which people don't want to turn off very often have a lot wrong with them.
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Decent, civilized adults protect little children when they sense something is wrong. Intelligent parents watch their little children all the time. Trouble is, there are a lot who are not intelligent. They may be well educated, but as parents, they do not have the knowledge they should.

If a parents objects to what you've done, just say "Safety first!" get in your car, and leave. They'll think it over. Probably.