Hey, Megan!
There are a few ways to go between Orlando International Airport and
Walt Disney World. You'll find the main players in the game right now - Mears Connect and Sunshine Flyer - do not provide car seats for the trip to your
Disney Resorts Collection hotel, but they do allow them. If you end up bringing one, you will need to make a reservation for the car seat, even if the child is under 3.
If you do a Google search, there are other options available for transportation that can include a car seat. Personally, I have used MCO Black Taxi, but only once. We had a positive experience, with clean car seats provided at no additional charge.
Florida law doesn't require the use of car seats in pretty much any type of chauffeur-driven transportation, on motor coaches, or in vehicles where the driver is compensated - think rideshare services. Keep in mind, though, that some rideshare service drivers won't take passengers with children needing car seats unless those passengers provide them.
Once you get to Walt Disney World,
Disney transportation doesn't require car seats. Bringing your own isn't an option, primarily because there are no seatbelts on the available modes of transportation to secure the car seat.
Lastly, consider renting a car. Because I have three grandchildren under the age of four, this is always a good option for us. All of the rental car companies I've worked with do rent car seats at a flat fee for your rental period.
I hope we've found an option that works for you, Megan!
Anna