If you are using the Magical Express to/from the airport and using Disney bus system while you're on property, you do not have to use a carseat. I'm not sure what the law is on cabs.
Our daughter is 3.5.....we only took the bottom part of her booster seat and the only time we used it was one day when my dad rented a car and we went off property. I didn't see one person using carseats on any of the Disney transportation. People with babies just held them on their laps.