Hi RuthieT!
I would say that it would depend on how much driving you intend to do; if you think that you are going to want to do some grocery shopping, try some restaurants not in walking distance to the parks or even visit the beach during the time that you're here- then renting a car might be the best option for you and your daughter. I think that most rental car companies have car seats that you can rent [usually run about $7/day].
Taking a shuttle van from the airport to the Disney Resort is probably going to cost about $25 [roundtrip] for adults and $15 [roundtrip] for children.
A taxi would probably run about $35 [roundtrip].
Sometimes, rental car companies in the area have great rates- so as long as the hotel that you're staying in doesn't charge you for parking, it might be as cost efficient [and convenient!] to just rent a car.
I hope that this helps! Have a great trip!
Lorealle