Its not required but the FAA, TSA and Amer Academy of Pediatrics all recommend using a carseat on aircraft until the 40lbs mark.
We are past carseat age but we used them for air travel until about age 3 - 3 1/2 depending on the child mostly because we felt safer and needed them for ground transportation anyway. If your using ME and exclusively WDW transportation then I think it would be very tempting to leave the seat behind as you can't use it on the buses. If your going to need one for a rental then you may as well use it in the aircraft as the child will be safer as will the integrity of the carseat.
TJ