Honestly -- I wouldn't check them at all. Car seat bags/covers/plastic bags do little to protect the seats from damage. The below video shows what happens to checked car seats:
http://www.youtube.com/watch?v=lzmJr1a-BHU
The pile of items to the left of the belt, that the handler is throwing more items onto, is a pile of carseats. Yikes!
The best case scenario after checking luggage is that there is *visible* damage so that you know you need to replace the restraint -- worst case is that there is *invisible* damage and the seat fails to protect your child in the event of a crash.
When I am doing carseat checks with parents (I'm a CPST,) and am determining whether the child's safe is safe for continued use, my list includes, "Has your child's seat ever been... dropped, thrown, checked on a plane..." It just subjects the seat to damage and stresses it is not intended to endure (outside of crash situations, after which it should be replaced.) Not a gamble I'm willing to take with my child's life.
What is the age of the child in the carseat? Assuming the ages of the children in your sig are correct and it's your 3yo, I would carry the seat on the plane with you and use it. For the booster, if it is backless, I would carry it on the plane and stow it in the overhead bin. If it has a back, you can pack the back, well-padded, in luggage. You can strap the car seat to a rolling luggage cart, or attach it to a rolling carryon with something like a Travelling Toddler or Go-Go Kidz Travelmate, allowing your youngest child to ride in it through the airport.
I know that's not the info you asked for -- but hopefully you understand why I provided it and it is helpful to you.
