Hi Ellen,
There are countless books/videos and other media related to Disney trip planning.
As a first step, I'd order the
free Disney Parks planning video. While this is made for adults, your granddaughters will certainly enjoy seeing video of all the rides and attractions they'll encounter during their visit. You should also make sure that they have videos or story books of many of the Disney films. My list would include Beauty & the Beast, Little Mermaid, Peter Pan, Cinderella, Toy Story, Lion King, and Aladdin as a good starting point.
There are countless interactive books related to Disney stories. I searched Amazon for "Disney Interactive Books" in the "Children's Activity Books" section and came up with hundreds of options.
There are several good videos about the Disney parks, but they might skew a bit old for your grandchildren. Look for "Disney Parks: The Secrets, Stories and Magic Behind the Scenes," "Modern Marvels - Walt Disney World," and "Undiscovered Disney Parks" for ideas. The DVD "Singalong Songs Disneyland Fun" is obviously about Disneyland, not Disney World, but there's enough overlap to make it work for little kids. And the entire Disney Singalong Songs series is great fun.
There are several Disney related cookbooks you could buy to work on projects with the girls before their trip. Try "Disney Princess Cupcakes and More" as a starting point.
Good luck with your planning. Please let us know what else we can do to help.