There's a dinner called Cinderella's Gala Feast. It's held at 1900 Park Fare in the Grand Floridian. It's an All-You-Can-Eat-Buffet hosted by Cindy along with Prince Charming, Fairy Godmother and Susy and Perla (the mice from the movie).
We ate at 1900 PF in June and the food was fine, good pizza and great bread pudding, but the character interaction was fabulous! The best ever character experience! I hope they keep it up.
At the Grand Floridian's 1900 Park Fare you will definately see Cinderella at Dinner.
If instead you choose to have Lunch or Dinner in the castle at MK quite often Cinderella is in the foyer greeting people ~ but that is not guaranteed. Fairy God Mother and Prince Charming can also be greeting guests and at time there is no one there at all.