I put a lot of research into this subject, based on some of the vehicle accidents I have been to during my day job. I looked into the CARES product, and eventually came to the conclusion that I wanted something more substantial. I was able to find a few accidents where the CARES harnesses either broke or allowed the child to slip, albeit not all the way out of the harness, but enough so that many of the resultant injuries were probably greater. The problem with the harness I found was that the rear seats in the early model Cessna aircraft don't have a lock keeping the seat back from folding forward during a wreck, thus causing problems. I agree with many of these posts, that a booster seat is absolutely necessary for the kids if a full-back seat isn't used. When I researched the seats, I decided to go with the Diono brand seats (I don't remember what model), due to their ratings and also the measurements of the seat. Two seats will go side-by-side in the backseat of my early 172, as the seats don't have a lot of the plastic edge moldings as many of the new generation seats do. They are reasonably light and my five year old, who is tall for his age fits just fine, with lots of room to grow into the seat. Both of my kids like the added benefit of sitting high enough to look out the side windows. If someone needs more info please feel free to send me a message and I will do my best to help.
Jeff