Hi, Pamela!
What a wonderful trip you will all have wandering the World! Have lots of room on your camera's memory card; you're going to need it for all the great photo ops together!
This very same situation has happened to me on several occasions. First, I'd call Walt Disney World Dining at (407) WDW-DINE, and ask them to add the additional guests to your reservation. If it's not available, ask if they can be a little creative. For example, I managed to get two tables of four when one table of eight was not available. Although the reservations were about fifteen minutes apart, I was told to tell the Cast Member at the restaurant's check-in desk that we'd like to eat together. They'll tell you that you might wait a few extra minutes so that two tables nearby become available, but they'll try to accommodate your wishes whenever possible.
Another situation arose when my Fairy Godfriend's daughter, an active-duty Marine, briefly home from war, surprised her mom by showing up at Epcot. (I knew that this was coming a day or so before it happened, but didn't know which day she'd make it there.) I dashed off to the restaurant podium personally and explained the situation to the Cast Member there, who didn't bat an eyelash and added one to our table.
In short? Call Dining. No luck? Ask at the podium personally. If waiting until the day of the meal unnerves you, though, you might want to consider some of the more off-the-beaten path character meals, which may have more availability than those in the parks. Check out all of the
Character Dining experiences listed on Walt Disney World.com.
Best of luck! Enjoy your great trip together, and be sure to get back to me with any additional questions you may have! I love to help!