When I grew up we had a flip up seat in the back of the station wagon bed that allowed 2 kids to sit in it facing the rear window. I also can remember many trips standing in the back of a van trying to balance around corners like on a surf board. And I remember riding home from the beach a couple miles sitting on the folded down tailgate of a pickup.
No shoulder belts or air bags, how did I ever survive? :crikey:
I also never wore a bike helmet, never owned elbow pads, and never a leash on at malls.
I did have a cup however.
I think if you look at the fact there are shoulder seat belts, you are in a massive tall truck with a steel frame, there is space between the rear and the rear seats, you would have far less to worry about than say the back seat of a van or anywhere in a sedan.