Here's the deal - car seats are not
required by the FAA but they
are strongly encouraged for children under 40lbs (or 44lbs if using the new CARES harness).
FAA site: Child Safety on Airplanes
Informational site with links to other sites:
Airline Travel and Child Safety fyi: this site also has tips on installing a car seat on an airplane for those who are interested.
If you're using DME and disney transport, you will not need a car seat on the ground. We did this on our last trip and did not bring a seat for our almost 5yr old...we did end up needing at least a booster for him (he still rides in a 5pt at home) because one of our flights was canceled. I was kicking myself all the way to the hotel and back to the airport - riding in a van w/seat belts - because he was riding in just the seat belt which is very unsafe for a 40lbs child. It was a totally unplanned for event.
Wendybird, the faq on
www.kidsflysafe.com addresses your question about the tray table and the CARES strap that goes around the back of the airplane seat.
If two adults are traveling together, you can put a front facing car seat in between them, but I don't think they'd allow a r/fing seat because it would block the window seat passenger from getting out of the aisle in an emergency. I do believe all three passengers have to be in the same party for the airline to allow the seat to be in the middle. We never tested this though - we just put the seat in the window seat since we didn't want to have to fuss over it, or be told to move it (and I wouldn't argue over something like that with an FA anyway).
hth