When we flew to WDW with our DS who was 2 at the time, we brought his car seat. Yes, it was a pain to lug around in the airport, but I was so glad we brought it. He fell asleep on both flights almost immediately!
We're leaving in 24 days and now we have 3 kids - DS8, DD5, and DD2. We checked into bringing a booster-seat instead of the carseat, but found that it isn't FAA approved. So, we'll be bringing that carseat this time, too.