Welcome BACK, Robin!
First off, a super double-ears cheers and Happy Birthday to your Father-in-law! I love that everyone is coming together to celebrate his birthday, and I love even more that you're all meeting at Walt Disney World Resort!
Walt Disney World Resort has a number of options available for all kinds of special meals, ranging from a small and intimate affair to staging a special convention event for thousands. I've personally eaten dinner in a semi-private side alcove of the upstairs dining room of Yak & Yeti Restaurant in Disney's Animal Kingdom to a hidden meeting room located above Frozen Ever After at the Norway Pavilion in Epcot's World Showcase. So really the sky is the limit in terms of how you'd like to celebrate.
Your best option for dining at a
Table-Service Dining Location where everyone would order off the regular menu would be to call Disney's Dining Reservation phone line at (407) 939-1947. If you'd like to set up a private dining experience, like a special pre-fix menu at a Table-Service Dining Location, or a buffet or plated meal in a small convention room or hospitality suite at a
Disney Resort Hotel, you will want to speak with someone from Disney's Group Getaways or Disney's Meeting and Events teams. For a group your size, you'll call the main Disney Reservation Center phone number at (407) W-Disney, which is (407) 934-7639. Let the helpful Reservation Cast Member know if you'd like a meal in an established restaurant, or if you'd like to create a private experience elsewhere, and they'll connect you to the right team who can help.
I hope this helped, Robin, and We'll See You Real Soon!
~Kristen