In reply to ebonyandivory:
You are asking about the legality, yet debating the safety and methods. That's a little weird.
Bottom line, it's not legal. Modifying the safety restraint system on a vehicle is not legal, and neither is using an after market product clearly designated as "not for road use". It is also not legal to use an OEM restraint system in ANY manner other than the original design. But no one is going to arrest you or take you to jail.
Does that really matter? Your real question is "Is it safe, better than what I've got, or what is the safest way to do what I'd like to do".
That's a different question.
If you modify the system then sell the car and someone is hurt, you will get sued (whether or not your system is better), and you will probably loose. If you ride a passenger who relys on the system and is hurt, you will get sued, and probably loose.
But I seriously doubt your kids are gonna sue you.
There is no law against something IN ADDITION to the original system (unless it interferes with the system). Leave the original lap belt, and add shoulder harnesses to make yourself feel better. If they fail, you are still relying on the original restraint system.
There is no way to do what you are asking legally. But there will never be a citation issued, and there are ways to do it safely.
I may have missed it, but how old are the kids? I wouldn't install a child seat relying on an after market modification.
One more thing... Are you prepared for the consequences? Sometimes, people die in accidents, regardless of the safety equipment. If your system were to fail, could you sleep at night? If your system did not seemingly fail, but your child died, would you feel it might have been your fault? It's nice to think we can "improve" a system, but when we do, we are taking the responsibility on ourselves, and need to be prepared to accept the full responsibility and consequences.