Here is what I did for my kids in the back seat. I have two boys 8 and 7.
I put a "eye" in the package tray behind the speakers, and purchased something like this for a school bus. It attaches to the eye on top, and the lap belts go thru a loop at the bottom. The eye I purchased was from a 2000 honda, my daily driver at the time. It was their car seat hook.
http://www.ezonpro.com/products/schoolBus/kidCam.shtml
It is like a 4 point harness.
Because it is made for a bus it is still a little long for my kids, 8 and 7.
I purchased mine here as they were local to me.
http://www.a-zbusparts.com/