I know that this is a serious consideration, but, I can't help but think back to the days when I was a kid. Car seats, like we know them now, did not exist. In fact, seat belts didn't exist. I remember the car seat that my sister sat in as a child. It had hook style back brackets that draped over the FRONT seat back and that was the only method of securing them. It faced forward and had a plastic steering wheel right in front of your face so you could pretend to be steering the car. I used to love that feature.
It was bad enough with a four door, but, imagine that with a two door car with seats that folded forward. They didn't have any latch on them to stop them from doing just that if you had to stop fast.
Once you grew out of the car seat (about age 2) you were then made like the rest of the passengers and just sat on the seat. (remember no seat belts) If the child was normal sized (small) they would usually just fold their legs under them to lift them up a few inches so they could see out the window or lay across the shelf behind the back seat just in front of the rear window. Yet, I know of no one ever injured by that little situation. I know that there must have been but I never knew any personally.
The only real constraint that you had was your parents arm that was placed in front of you during a fast stop to prevent you from flying through the front windshield. Ah, the good old days.