[Moppie]

Very easy, you could simply wire it into one of the existing systems that use GPS to track stolen cars.

A sensor in the seat to tell if its occupied, and another sensor in the car for the heat.
Set up a small circut so that if the seat is occupied, and the car is to hot, the alarm is activated, and the central locking unlocks the doors.


Couple of problems though, the sort of people who leave children un-attneded in car parks don't usualy have the sort of incomed needed, or the thought to install such a system.
No car manufactor is going to fit as standard to new cars because of the increase in cost, vs the small benifit to a small number of people.