I would suggest renting a budget economy car.
Yeah the Magical Express is good, but its nice to sometimes have a car as well, in case you might want to go visit the coast in Tampa or Cape Canaveral and see the Cape Canaveral National Seashore or the Kennedy Space center. or go to Tampa-Clearwater for a day and see the Gulf of Mexico.
its also good to have a car, just in case you don't want to wait for one of the disney shuttle busses. I found the busses unreliable, and it took awhile to wait for them at the All-Star Music Resort.
The resort itself was great, but I waited like 20 min for a shuttle bus once.
A couple times I just gave up waiting and just drove to the park . it wasn't a very long drive from on -property.
The other good thing about having a car is if you need to get anything for your hotel room , you can drive to the wal-mart in Kissimmee and pick it up. its not too far a drive ..
Lots of services in Kissimmee which wouldn't be accessible if you didn't have a car.
I woudl suggest renting from a reputable company like Alamo/National, Dollar , thrifty,
Avis, Hertz, or Budget.
Don't fall for discount rental companies like Payless, etc.
Keep in mind that rental return times vary and allow at least an hour extra when returning your car the day of the flight back home.
I almost missed my Jetblue flight to Boston because of the line at the return counter.
and this was in the off-season (MAY)
arrive at the airport 4-5 hours before your flight for orlando departure.
the line at the security was insanely long! and SLOW
if it wasn't for the JETBLUE employee letting me through the employee entrance I would have missed my flight, and would have had to fly to New York and take a bus back to boston.
More info on renting a car in disney
A good back route to the airport would be Palm Parkway from the WDW Complex
to Central Florida PKWY
to 92 N and then onto Tradeport Dr. into the Airport.
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Hope this helps with your decision.
(for a larger family you can always rent a minivan)
Sincerely,
Jeff D.
Winchester, MA