3 week trip with kids, help me with logistics- boosters and bags for the kids

[Reader]

We will be renting a car to get to the airport 100 miles away (cheaper than leaving the car in the lot for 3 weeks), then flying, then renting a car for our destination. When we get there we'll have 1000 miles of driving to do, so I want safe carseat options. My kids are 9, 8, & 4, and they currently ride in Britax Frontiers & a Sunshine Radian (small kids). We have an extra Radian that the 8 year old still fits in. We've never needed carseats for a long trip at our destination, so I'm not sure how to work this. They can't use boosters on the flight, so should I try to carry 2 Radians and a booster on? We'll be loaded down with 2 laptops and a CPAP, too, so I'm not sure how we'll carry all this on. Maybe we should buy new seats when we get there. If so, what?

An additional question is what's your favorite bag for older kids to take with them on the plane? They have just used backpacks in the past, but I'm wondering if I should get them backpacks with wheels.

[hillview]

I'd put the 8 year old in a booster and carry on 2 boosters and a radian (make it a backpack). WRT a good bag for the kids I'd stick with backpacks. We have roller backpacks but the issue is when they don't want to carry them you end up with a lot to drag along. Good luck!