ERIN: - Deep breaths. I'm confident this will work out for you.
I had a similiar problem with our trip last year when we went with my sister and her family. Granted, we weren't as big a group as yours will be - we were only a party of 9. But for every one of my ADRs, I was told they couldn't fit us all at one table - always had to book a table of four and a table of six. And they couldn't guarantee we'd be close to each other. Sometimes my ADRs for each table weren't even for the same time. For instance, our breakfast at Norway had our table for 4 at 8:40am and our table for six at 9am.
BUT every single time, once we got to restaurant check in, we were accommodated. I never had to argue for it or put up a fuss. The hostesses always made it seem like it was no big deal and "of course, they'll keep us together."
Keep in mind, the ADR people are just a central booking line. I bet half those people you talk to have never even stepped foot into a lot of the restaurants they are making reservations for. They just have to give standard, scripted responses. Just get the ADRs for the 21 people and make sure you show up a little early. You may have to wait a little longer but I'm sure they'll accommodate you. Being that it's a child's birthday celebration, I can't imagine that they wouldn't. It's Disney, afterall.
Now, I'll go back and respond to the other comments once I get Amanda off to school. I just saw Erin's dilemna and wanted to get my thoughts in. Hope everyone has a good morning.