Hi Christine,
Welcome to the Disney Parks Moms Panel. My family absolutely loves dining at
Cinderella's Royal Table. The castle experience is magical! There is no guarantee that the Cast Members at the restaurant would be able to add your mom on the day of the dining experience. I strongly recommend that you contact Disney Dining at 407-939-1947 to see if they can add her to the reservation. If they cannot, you should definitely request to add her when you arrive. I do suggest that you have a back-up plan just in case they cannot accommodate her.
On one of our visits to the
Walt Disney World Resort, we were unable to secure a reservation at Cinderella's Royal Table for our entire party. My cousin took the girls to Cinderella's Royal Table while my mom, her sister and I ate lunch at a restaurant nearby. It certainly was not ideal, but we made it work.
Keep checking
Disney Dining availability prior to your arrival. They sometimes have cancellations.
I am sending some extra pixie dust your way so that your mom can enjoy Cinderella's Royal Table with the family!
-Donyell