Unfortunately two local toddlers have died recently after being forgotten and left in cars when the outside temperatures were triple-digits. A thought came to my mind.
Earlier this year I went to Home Depot and bought a two-gallon can of texture paint. The texture in the paint adds to the weight. I placed the can on the front seat. Periodically the seat belt light would turn on because the sensor detected the weight and the fact that the seat belt wasn't buckled.
One solution to preventing these deaths is to also wire the rear seats, as that is where small children sit. After the car's engine is turned off and doors have opened and closed, the sensor will notice a seat belt buckled. After 5 seconds the horn will start sounding. Someone will notice.