I like part of your idea.

The idiots who leave their children in a hot car "I'll only be inside for a minute" would not pay attention to an alarm of any sort. Senseless, and stupid, and you're right, it happens every year.

Then again, if you wired this up to dial a 911 call and get the police there to haul the parent to jail for child abuse, then I would be in support of this!

Um, how does your sensor know the difference between a child and a bowling ball? Or a bag of groceries? Or a suitcase? (These are all things that I have tossed in the back seat from time to time.)

The truck I drive at work has sensors that alert me when one of the passengers does not have a seat belt on. It was annoying as hell to have to have to buckle the seat belts if I was hauling equipment in the back seat. I say was because we discovered the manufacturer included a code to disable the alarm. The owner's manual made no mention of the code but there are plenty of "how to's" on the internet.

If occupant alarms that went off when the vehicle was parked were to be installed I would bet on most people disabling them in short order.

Not idiots- just people. The people who leave their children and infants in car are typically like most people- caring, doting parents who are just tired or had a change in routine that caused them to forget that their child was in the car. Generally, most parents of infants and babies are sleep deprived and more likely to make mistakes- tragically this is a mistake can't be reversed and can end the life of a child. As a parent of an infant I consider myself simply lucky that my chronic sleep deprivation has not led to anything like this. Still think this only happens to bad people- read Gene Weingarten's piece on these tragic events in the Washington Post Magazine/March7, 2009.

These days, it's illegal to put a small child in a car without a special child safety seat. Any kid big enough to not require at least a booster seat can easily open the door and get out themselves. Thus, it would only be necessary to put the sensor in the child's safety seat. You shouldn't be putting briefcases, groceries, etc. in the safety seat anyway. This would eliminate the need to install the system directly into the car, saving money for those of us with no children. It would also allow you to use them in older cars that don't come with such a system. The only problem would be how the seat would know when you are getting out of the car. It might be possible to install a small transmitter that senses when the car is shut off by the drop in voltage from the alternator. This could be detected anywhere in the car's electrical system. For example, it could just plug into the cigarette lighter socket. Then, when the car is turned off, it will transmit a signal to the seat. Instead of a transmitter, you could just have a long cord and plug the seat directly into the socket. It would be better to use the pin switch in the door that turns on the dome light and other warning devices, but that would require more installation skill than the average Mom would have. There may be better ways to do this that are not occurring to me just now.

Another issue is that many people leave children in their cars because they expect to be "right back". Then, they get side-tracked and forget to go back and get the child. It might be possible to have a device that calls your cell phone if it thinks you have left a child in your car. If you don't answer, it could call someone else. This might cost a lot though, so people might not be willing to buy it.