Buses are designed differently than cars and the laws of physics help keep you safe.
Millions of children have ridden and do ride buses everyday around the world and at Disney and are safe. Mine always loved being "big" and riding at Disney without it.
If you aren't renting a car I wouldn't lug a car seat thru the airport and have it take up room in the room. My kids never used a car seat on the plane and have flown tens of thousands of miles without incident.
http://www.google.com/images?oe=utf...esult_group&ct=title&resnum=9&ved=0CFIQsAQwCA
http://www.kmbc.com/news/24295018/detail.html
http://www.msnbc.msn.com/id/37902364/ns/us_news/
All it takes is one accident you are on....
If its a bus inside Disney, usually the speeds are slower.. but if I were to do the ME from MCO, heck no... i'd rather have the carseat taking up space than a freak accident occuring. My DD5, I can convert her Marathon seat into a booster, which we'd bring (and check on the airplane), but the 2yo will have his seat.
But then again, I always get a car when I'm on vacation.. I like my freedom