@ AbuJamal!
Good to see someone who really cares about his family. Well have been through almost the same scenario about 2 years back.
If you can find something like what scorpion80 has mentioned that would be the best. Otherwise, you will have to do dasi style. This is what I did.
Find a good belt (seat belt or any other sort of belt). Get hooks on both ends (set of lock, just like a seat belt or any lock like airplane seats would be best). Cross one end through under the back of rear seat, tie or rap around the CNG cylinder holder. And then through (in between) the speakers board and top of back of rear seat. Now you have both the ends on the rear seat, place a baby car seat and cross the belt through the belt holder and tie it real good.
I hope you understood what I have said.
One more thing, if your child has the habit of sitting in lap, initially he/she could give you hard time. No matter what you have to ignore the child's protest. (In my case, I made my child sit in his seat since the day he was able to hold his neck himself, so there was no problem, in fact he loves sitting in his seat.).