Hi Sheri,
I am sending you big amounts of pixie dust for an amazing trip especially with Grandpa. First, you want to call Disney Dining and ask for the Grand Gathering options. They will be able to assist you finding a location which can accommodate your group. You will need to split into a few tables however they are usually placed next to one another. Here is a
link to more information on Grand Gatherings.
I think you will have more luck doing a lunch, early dinner or late dinner with your party size. If I was picking amazing, I might go to Jiko or California Grill for the overall dining experience from food to service to environment.
If I am going amazing fun, I might go to the Whispering Canyon Cafe which is delightful or to 1900 Park Fare for dinner with Cinderella and her family. 1900 Park Fare was a highlight of my last trip as it was so much fun, and I have eaten at many a Disney restaurants.
You may still have a good chance at a character meal especially if you think outside the box-Tusker House in Animal Kingdom or Cape May Cafe at the Beach Club.
Wishing you a magical vacation!
Beth