I would suggest either 1900 Park Fare (Grand Floridian) or Chef Mickey's (Contemporary) for dinner your first night. 1900 Park Fare has Cinderella, the Prince, the wicked Stepmother and the Stepsisters. Chef Mickey's has Mickey, Minnie, Goofy, Pluto and Donald. I would pick the one that has the characters you/your grand-daughters would most like to see.
Another first-night option is the Hoop Dee Doo Musical Revue. It is a dinner show that is fun for all ages. The meal includes ribs, fried chicken, beans, corn, salad and strawberry shortcake. It has been running since WDW opened and remains as popular as it was then. If you did the earliest show of the Hoop Dee Doo you could follow it up with the nightly campfire (details to follow). There is also a Spirit of Aloha dinner show at the Polynesian resort. Personally, I do not think it is as good (neither the show nor the food) as the Hoop Dee Doo, but if it appeals more to you then it's worth trying.
There is a nightly campfire at Fort Wilderness. It features a visit from Chip & Dale, the ability to roast marshmallows/make s'mores and a sing along. It is followed by a Disney movie under the stars. This would be easiest to do if you have your own car as the movie ends late and it would take a while to get back to your resort using Disney's transportation. (The same is true for the late show of the Hoop Dee Doo...also located at Fort Wilderness.)
Finally, Cirque du Soleil offers a great show, La Nouba, at Downtown Disney. It is full of acrobats, aerialists and dancers and is a very visually striking show. I think even your young grand-daughters would enjoy it. There are lots of restaurants at Downtown Disney at which you could eat before or after the show.
As for other things to do...Cinderella's Royal Table is located in Cinderella Castle at the Magic Kingdom and offers princess character meals for breakfast, lunch and dinner. It is pricey, but offers a great experience...especially for princess lovers.
Be Our Guest is the newest full-service restaurant at the Magic Kingdom. It is in the Beast's castle. Reservations are hard to come by and available for dinner only. The only character there is the Beast. Personally, I would opt for Cinderella's Royal Table over Be Our Guest for your princess-loving girls.