That's why you check all around the car before setting off.

Unfortunately children playing in the yard have ample time to get behind the car between you checking it, entering the car, starting it, and reversing.

Yes, in principle one could take whatever measures necessary to prevent such accidents. In reality, backup cameras save lives. Just like seatbelts, anti-lock brakes, crash safety standards, and other safety features that "Real Manly Drivers" protested against back in the day.

Children tend to move around.