Um, if it detects an obstacle, how did it hit the trailer?
It has ultrasonic sensors in the bumpers. They're too far down to detect a trailer at windshield level, but they'll detect the legs of a person standing in front of it.
It doesn't guarantee to detect every obstacle every time.
> It doesn't guarantee to detect every obstacle every time.
In Silicon Valley, they call this "eventually safe".
Which is probably how it ended up rolling over your child in the (hypothetical) first place.
Assuming the driver did summon the car he must not have been paying much attention.