During my first drive with one of my daughters, I noticed she was struggling to hold onto the back seat and in particular in case of sudden braking. That is why I put belts in the rear seats. Purists will scream, but it does not matter anyway this assembly can be removed without a trace.
This kind of installation is not licensed, nor approvable. In case of accident your insurance company will cover you in any way.
Equipement: a piece of iron and two lap belts.