Depends on your definition of "best". The easiest would be to take a cab, but the cab may or may not have a car seat that will work. Second easiest is driving yourself, perhaps by renting a car across the street at the Wyndham. They probably can rent you a car seat as well. Or you can take a car seat with you just for this trip. A hassle, yes, but it's just one more checked bag for the pile.
However, via bus, yes walking or boating to DTD and getting a bus from there would be the easiest. Closest stop to Fantasia Gardens is Swan. Closest to Winter Summerland is Blizzard Beach.
It's not immediately obvious from either bus stop how to get to the miniature golf, though you can obviously ask a CM for directions and I think there are signs. I'd scope it out on a map first to familiarize yourself with the layout before you get there.