Back Seat Passenger Alert

I am concerned about the number of children dying from being left in the backseat of vehicles, most notably during hot weather.  Parents/caregivers are exhausted and going about life in a fog!  Could there be an alert or alarm that goes off once the engine is turned off to alert the driver that there is a passenger in the backseat?  The alert/alarm could stop when 1) the backseat seatbelt is unfastened or 2) a button is pushed by the driver or 3) the weight is taken off of the seat.   Thank you for any consideration of this idea.
Tyson S 08/01/2013
What if I have a box in the back seat that i secured with the seat belt. Would the alarm go off, with your idea it would. It wold make carrying anything in the back seat a hassel. Pain in the butt, and people like that who do that shoul not have kids