I don't think you'll have a problem - sometimes they check and sometimes they don't. But I am confused though, if your kids won't sit in them, are you wanting to install their car seats in their airline seats, and have them sit on your laps? If you don't intend for your kids to use them, and they do ask to see the FAA sticker, then you'll have no choice but to gate check if they refuse to allow them to be on plane. Otherwise, there is absolutely nowhere to store them while on the plane - they will make you gate check them (which is a bit safer at times then the luggage check).
Good luck, Tiger