> Could cars could be smarter? I wonder how hard it would be for them to detect the presence of a child still inside?

The passenger seat belt alarm on a car I had rcenetly would trigger considtently with a gallon bottle of milk; to detect a child in a child seat you'd need it to ignore the seat but trigger with child plus seat, which may be a little more involved but shouldn't be impractical.

Alarm UX may be tricky: you ideally want to alarm when the driver exits the car with a child in the back, but to avoid alarm fatigue you probably don't want to do it when entering.