re: Idea for a car safety feature

My thing is, we have a sensor to sound an annoying ding to have a passenger put their seatbelt on, but we can't come up with something to sense that there is a child in the car??? My damn sensor goes off if I put a bag of fast food in the passenger seat sometimes. We can't figure out to have something that can tell if a child is in the back seat and to beep at you like when you leave your lights on.