Have you thought about something like the Gogokidz?
http://www.toysrus.com/product/index.jsp?productId=265882...
They aren't cheap, but WELL worth it.
When my son was four and five I flew cross country with him twice by myself. It was SO nice to be able to just buckle him in his carseat and pull him through the airport.
Even if you don't actually pull the kids in them, it makes the seat really easy to pull along with you.
As for the seats in the row... I think the FAA says carseats have to be in the window seat (so that in an emergency no one has to get by it). If that's the case, you'd have to split up and put each one in a window.
The 5 year old could comfortably ride without the carseat on the plane at this point. Perhaps gatecheck her seat (somehow I trust that more than checking because it goes on last and comes off first).
There are also carseat bags you can buy to protect the fabric from rips or stains when you check.
Hope this helps.
T.