Mid/rear Seats Assembly problems of the 2010 Toyota Prius

My car was rear ended by a sedan and my car was pushed forward and ended up rear ending a car in front. Not sure about the speed at which car rear ended me, but I should have been traveling around 10 miles or less. At the end I noticed that my car seat had slid back completely. I expected the car seat to hold steady so that the belt can be effective. I have lower back and right shoulder pain and not sure if it would have been prevented should the seat have stayed in place. I think this is a defect that needs to be addressed and not sure if it's specific to my car seat. My kid was in the back seat and luckily there was enough room and the car seat moving back didn't hurt her. But she was shocked that the car seat came close to her at the of the accident. Both my kids in the back seat complained initially due to pain and shock. Now they seem to be fine now.