I looked at this several years ago. I looked at makling a pad that attached to the metal structure behind the plastic trim below the window and provided head cusioning since my kids bonked their heads on the trim or window occasionally. The rear seat is so vertical, there was no way to avoid bonking your head.

But, to me, it looked like any padding would cause an even more uncomfortable seating position, pushing the head a bit forward.

Find a kid seat that fits well. Get larger ones as they grow. As they get older, they won't want to ride in the back anyway due to the discomfort of the seat shape.