Good idea. I don't understand how this happens so many times. I would think people would remember they left another person behind in the car, without needing to be reminded, but if such a gadget will help them remember, then I am all for it.
I don't understand why a car manufacturer hasn't come up with this already. They could market it as the "safe car" and have a kid feature you could turn off. Snap the car seat (and they could sell those too) into a thing that shrieks if if is still snapped in when you open the driver's door.
We do not have children, just pets. We have a quiet elderly dog that rides in the back in a cage when we go hiking (we live next to the Gettysburg battlefield). I am terrified I might forget him and leave him in the hot garage, and go out of my way to remember him.
What a sad story.