S.H.
I recently got my daughter the Graco Nautilus 3-in-1. It holds them in a 5-point harness until they are 65 lbs. Then it is a belt-positioning booster, THEN the back comes off & it's a backless booster. You don't say how old or how big your children are, but keeping them in the 5-point harness as long as the car seat allows is best. My daughter is only about 40 lbs right now & almost 5 years old. I estimate she'll be in a 5-point harness until... high school?? haha (Dad & I are both petite, with me only 5 ft tall)