Help picking new FF seat

We'll need to buy two new seats for 5.5 year old DD in the next year or so, and might go ahead and buy one of the seats sooner rather than later so that my parents can each have a seat in their cars and don't have to switch the seat back and forth. DD is 5.5, a little over 45lb, and just recently turned FF in her two Radians. Once our twin girls (just born in May) move out of their infant seats we are going to pass both Radians down and get new seats for the older DD.

I'm not set on a FF/booster, since a RF/FF seat could be later used by the little girls in my parents' cars, and booster seats are fairly inexpensive. I'm not sure, strategy-wise and cost-wise, which would be better, so I'm open to either. It would be nice to get two of the same seat for both our car and my parents' cars, but not necessary.

My parents drive a GMC Sierra and Acura RDX and we drive a Ford Focus. The only thing I'm looking for in my parents seat is an easy seat to install so that they can move the seat between cars themselves! With our Focus, the main thing that we need is narrow! Aside from that, I would really like to keep the cost down this time. Last time we went for two Radians to keep DD RF longer, but now we just need the 65lb FF limit.

What seats would you buy in this situation?

Also, in writing this out I just realized that I haven't thought about what age she might start riding in a booster, and if she could start going in a booster in my parents' cars within a year or so...

My DD started using a booster at 6. I had to re-harness her a few times when she started acting silly and not sitting correctly. She's been primarily in a booster since 6.5 or 7 now. I let DS (5.5) ride in boosters on occasion and he does well so far. I think he's heard all the reminders I've given DD so much that he remembers them well and sits well. I know if he was boostered full time, he'd get wild sometimes so he is in a Frontier 85 full time in our van. You could certainly try a booster in your parent's and see how she does. I'm in the US so I'm not sure which current seats would be good for her. We have the Evenflo SureRide here and it may be going to Canada. Depending on it's limits there, it'd be a good option to ff in for a year then pass down to rf a baby.

My DD is also 5.5 and she rides in a booster in both of her Grandparents vehicles. I used one of the boosters in my van for about a month to train her, and then felt confident in her using one in their cars occationally. She is still harnessed in a FR85 in my van.

One perk of her using a booster with the Grandparents that I hadn't thought about in advance, is that I usually did the installs for them prior as they just couldn't ever get their convertible seats in that well (or didn't try hard enough because 'their generation' just didn't worry so much about this stuff ). A booster is so much easier for *them* to use properly and safely every single time, that I feel much better about it. In an emergency if I wasn't around to install their seat, I know DD would still be riding safely.

How tall is your DD? Do you know her torso size? If she is tall you may not have a ton of convertible seats left to choose from that will be narrow, and give her growing room. Maybe a Complete Air? But I'm not sure about the harness height off the top of my head.....