Hi, we took my daughter to Disney when she was 3. You totally do not need it for the shuttles - she just sat on our laps or the seat between Daddy & I.
Your choices, don't deal with the carseat at all and leave it at home OR bring it for the airplane ride. Kids know they have to sit down in their carseat and can't get up! We are big travelers, and we didn't bring it when she was between 2 and 3, and alls she wanted to do was walk up and down the aisle, etc., etc!! You can only do that a couple times before the Flight Attendants get mad. If you don't want to bring the big, bulky carseat - you can even improvise and bring a booster!
Best of luck & have fun!!!! We are going again in May (my kids will be 6-1/2 and 2-1/2).
Take care!
M.