I would recommend renting a car, but here is what I know.
They have had a shuttle that costs $7 per person for a roundtrip including kids. A family of four would be $28, for example. Times were limited and you used to need a reservation, but I don't know if that is the case right now. I would double-check this info with the resort as things are always changing.
If I didn't want to drive I would use Uber/Lyft, which can take you directly to all the parks except MK and all the resorts and at the times that you want to go.
Obviously, the more you plan to move around and come and go from WBC the better a deal renting a car will be as multiple rides will add up. You also have to pay for transportation to/from the airport.
For example, our family of five would have to pay $35 to ride the shuttle. We like to go to one park in the morning, return to the resort at midday, then go to another park in the evening. In that scenario, it would cost us $70 for shuttle rides.
Having a rental also can save you money by making it easier to eat off-site, go to the grocery store, etc.