Hi Laura!
Cinderella's Royal Table inside Cinderella Castle in the Magic Kingdom is one of the hardest reservations to get. Unfortunately, it generally books up close to 180 days in advance. While you can try walking up on the day of your visit, the probability of success is very small. Another option is to try calling every morning at (407) WDW-DINE to see if any guests have canceled their reservations.
However, another great option is Cinderella's Happily Every After Dinner at
1900 Park Fare in the Grand Floridian. It's a buffet and your granddaughters can meet Cinderella, Prince Charming, and the Wicked Stepsisters. It's a lot of fun, especially interacting with the Wicked Stepsisters.
She also sometimes appears at the Princess Storybook Dining in the Norway Pavilion in Epcot. However, it is not guaranteed. Other Princesses that you may encounter there include Belle, Princess Aurora, Snow White, and Jasmine.
Have a magical vacation!