Hello, Jennifer,
It sounds like you are planning an amazing trip to Walt Disney World! I cherish the multi-generational trips we've taken as a family and have such wonderful memories of all the fun we had together and wish the same for you. Since you are needing to modify a dining reservation, I'd suggest you first go online to change the reservation. You'll need to be
logged-on to your My Disney Experience and click on the "My Plan, Reservations, and Tickets." Once there, you should see all of your dining reservations and the option to change them (on the right side of the page). It is a pretty easy process (just update the number of guests), and if there isn't availability when you first check it, I'd encourage you to check back every so often. If you are unable to change it after multiple tries and are willing to try a different restaurant, you could certainly do a
dining search for a party of 11 and see what is available. You may end up finding a new favorite restaurant you've never tried!
Another thing you can try, but is not guaranteed, is to ask about adding one more guest to your reservation when you check-in at the restaurant. The restaurant may be able to accommodate your request, but you'll want to have a back-up plan in case they can't.
Wishing you a magical trip! Please let us know if we can help with any further questions. -Kara