For lap belts, only child seats with thier own seatbelts will do. the older the kid is, the less of a chance you have of having a child seat with integral belt. most larger kids, starting at @3, go into some sort of booster that utilized the factory should belt. For them, only rear shoulder belts should be considered.

and before anyone pipes up and says "if you car didn't come with shoulder belts or any belts at all, you can't get a ticket" just don't.

I've been down this road before. I waited to let my kids go in the charger till I had a chance to install 3 point shoulder belts in the rear. Then I got into an accident with my 8 year old riding in the back. 12K in damage and she did not have scratch on her...because she was belted in properly with a proper belt.