I've just realized that DS2's seat in DH's car is expired. Luckily he rarely rides in it. DS1 has a Britax Frontier 90 in DH's car. I'll move DS2 into that seat but I can't decide what to replace it with for DS1. DS1 is 8.5 yo, 51 lbs and 48.25 inches tall. He occasionally rides in DH's car. My car is the main car for him. What should I be looking at as a replacement for DS1? TIA