http://www.safedrives.com/products.asp?cat=105
I wouldn't bother with the walmart padding. It might feel soft, but in a real accident it wont provide any real protection.
What I would suggest is dual density padding.
Normal SFI 45.1 padding is great, but it is designed to protect a helmeted head from being damaged, it won't do a bare noggin too much good. The dual density stuff is more spendy, but it should protect a wider range of things from incurring too much damage in an accident
Really though, I would try not to put too many people in the back seat, its just not safe back there with the cage, no matter what kind of padding you put back there.