Every car in this day and age in the US has to support near field communication tire pressure sensors. If a sensor detects low pressure or malfunctions, the car alerts the driver.

If we can put these cheap wireless sensors in car tires, there's no reason we can't require a weight sensor in car seats sold from 2018 onward that will sync up to cars (which can be required to support "pairing" with the car seat). If the car turns off and there's weight in the car seat, doors cannot be locked. If the internal car temperature gets hot and there's weight in the seat, alarms start going off.

Boom. A child never dies in a hot car again.

Just need the government safety organizations to write the regulations and it's done.