There's an even simpler (if imperfect) solution: Remind the driver to check the back seat if they opened one of the back doors before they got in the front sea

There's an even simpler (if imperfect) solution: Remind the driver to check the back seat if they opened one of the back doors before they got in the front seat. Heck, most cars have seatbelt+weight sensors. Just give a warning if you detect anyone under 100 lbs in the back.