I have no idea if we can gate check the 2 car seats we are planning on taking. Our local AA office can't tell us for sure either way - they say we'll have to check with the agents at the airport when we get there!

They said that as far as they knew, if the child is over 2, the seats would count as part of the baggage allowance, rather than be considered "personal items". GRRR. So, I have no choice but to assume I will have to check them as baggage, not at the gate, and wrap them up to suit. I think I will find some bubble wrap and wrap them in that first, then put them in a big garbage bag (we can't get the special car seat carrier bags here). It also means that we might have to carry the backless boosters instead of the 5 point harnesses (our girls are borderline weight, and can use both safely enough), so that we can fit them into 1 bag and have them count as just 1 piece of luggage. It will depend on how many other suitcases we have to carry - I'm shooting for just 3 between the 5 of us, which would mean that we could possibly wrap each harness seat up separately and count them as 2 pieces.
I guess what you can and can't do depends on the airport?