I think you are trying to solve the wrong problem. An alternative way to look at it is that the problem isn't leaving children in cars, it is that cars get too hot. It is basically a solar oven. Instead of trying to detect children, why not make it so that when the car is parked, that enough airflow occurs that the internal temperature of the car is similar to the outside temperature?

Of course, this solution has two problems. One is that you want your car to be sealed against outside air in cold climates. The airflow could be gated by a temperature sensor. The other is that sufficient airflow might make theft easier. This means that the airflow must be provide through a protected location (as opposed to e.g. just opening the windows).