Hi Julie,
Congratulations on your upcoming December wedding. Walt Disney World is a magical place to get married, indeed.
Although you would probably get a much more comprehensive list of options from the
extraordinary wedding planners at Walt Disney World, I know of a few restaurants that have semi-private or private rooms that might be good options for your wedding guests.
Although the California Grill is not considered casual dining, it does have a lovely private dining area with views of Cinderella Castle. I was part of a group of 20-25 there for dinner in December and it was a perfect fit! More casual options would be Cape May Cafe on the first floor of Disney's Beach Club Resort and The Wave at Disney's Contemporary Resort. Both of these restaurants have areas that are set aside, but not totally separate, from the main dining area. Although The Grand Floridian Cafe might not have a separate room for groups, it would be an elegant yet casual choice for your guests. One added benefit to all four of these restaurants is that they are not in the Disney theme parks so admission tickets are not required.
With a group as large as yours, advanced dining reservations will be critical. At a minimum, you'll want to call 180 days in advance of your arrival to make reservations. However, I highly recommend that you call the folks at Disney weddings (321) 939-4610 or at the Grand Gatherings office (407) WDW-PLAN to see what assistance they might provide with your exciting event.
Wishing you many magical years of marriage from the entire Walt Disney World Moms Panel!
Jill