Back in days when most people owned sedans and all sedans had a big shelf behind the rear seats covering the trunk, on long trips I would sometimes sleep up on that shelf (I was six or so). When I was nine my dad had a '58 Corvette. We went for a drive one day with my dad and his friend in the seats and myself and the friend's two boys sitting up on the rear deck with our feet behind the seats, holding onto the seatbacks so we wouldn't fall out. Of course there were the family outings with parents in the bench seat in the pickup truck and the kids all piled in the bed, occasionally standing up with arms on the roof of the cab.
Yes, we survived. Well, most of us, anyway.