My 8 yo, 51 lbs and on the short side and my 6 yo, 54 lbs and on the tall side (they are actually the same height), are both in the wide frontier seats. We just welcomed baby 5 and need to fit thee across in the 3rd row with a radian xt. Any suggestions on narrow boosters and advice on whether to go high back or backless? They are used to the comfort of the frontier s, but at the same time, we home school and do not spend a lot of time in the car. They are both asking for backless b/c they seem more grown up, but know that I get the final say. I'd love for these to be our last car seat purchases for these two kids. Thanks!