I've toured with a large group a time or two, Kelsey; let me assure you that you are going to have a great time! I understand your desire to plan a special meal for your mom. Honestly, there are a variety of wonderful choices that fit the bill; the tough thing is going to be narrowing it down to just one. That said, here are some suggestions:
During our most recent visit, we tried
Via Napoli for the first time. The food was amazing and our server (Simone) was attentive and fun. They do have accommodations for large groups to sit together as well.
Another great choice would be
'Ohana at Disney's Polynesian Resort. The name literally means family, so what better place to celebrate? Food is served family-style and is all-you-care-to-enjoy. While there are no characters at this meal, you will find plenty of entertainment in the dining room. If you're looking for a quiet experience, perhaps 'Ohana is not the best choice.
Lastly,
The Wave at Disney's Contemporary Resort offers fresh, flavorful food in a serene (though not stuffy) setting. There is plenty of space to accommodate large parties. Another bonus? The adults might enjoy a wine or beer flight with their meals.
Hopefully, one of these restaurants will be just the right spot to celebrate with your peeps!