We have a Frontier. My 6 yr old (45" and 43 lbs) uses it. It is very large, sturdy and seems like it should fit her a while longer. Will you be needing to fit 3 kids across in the back seat? If so, I highly recommend getting your son a harnessed seat - it's so difficult to buckle in a booster that's right next to a car seat! Your son is so light, I wouldn't like the idea of him being in a booster full-time until he reaches 40 lbs. so, I'd be looking first for a harnessed seat for him. I'm not sure if a Frontier is the best choice for 3 across, though (I'm going to have to cross that bridge soon as well, as we're expecting a third in April) - it's really pretty wide. I do love it as a harnessed, seat though!
My 6 yr old does use a Graco Turbobooster w/ Safety surround in our second vehicle. It's narrow, so I would also be surprised if larger kids would feel comfortable in it. It does seem nice and cozy for a smaller kid, though. I think Target has them on sale for $59 this week.
I've gotten slightly lost here. What kind of seat is your son in now? Is his seat expiring, is he outgrowing it, or do you just want to pass it on to the new baby (or to your 2 year old)? And your 2 year old daughter's seat is expiring before the end of the year as well?