I stopped using car seats for my kids at age 4 for airplane travel, and you cannot use booster seats on an airplane.
If you are going to use Disney transportation you won't need them at all since you will be travelling in busses. If you are renting a car, just have the car seats checked with your bags at the airport. They do not count as luggage. Although local laws may allow children not to be in car/booster seats, for safety purposes I always use them until they reach the age of not needing them anymore. You can never be too safe in a car, IMHO!