[ClimaxGravely]

Not the person you asked but if I had to guess it might be similar to how people react to leaving a dog in a car. That is to say it's a heat issue.

[Symbiote]

Unlike dogs, children can open doors and windows, and can shout for help if they think it essential.

[mitthrowaway2]

In many cars, power windows can't be operated when the car is off with the key removed.

[gnicholas]

Definitely true…but you can always open the doors. Kids that are 10, 11, or 12 are old enough to open a door.