I thought there is a very simply solution for newer cars.
If the car is turned off and locked, but a seat belt is still buckled, don’t lock the car, and instead sound the loud beep noise (the one that tells you you can’t lock the car because a door is open).
Sure, a few people might be mad because they leave their seat belts locked. But it would likely save lives. And the car has the technology already (well, they probably don’t have sensors for the back seat seat belts, but the technology to sense locked seat belts is.
The car’s seat belt secures the child chair to the car. You don’t unbuckle it to take the child out. The child chair has it’s own seat belts.