Sounds like you have quite a great group there. We have not been to Mickey's Backyard BBQ but as with anything Disney, I would assume it to be fun and accommodating to all. Here are a few sites that you might like to browse: Walt Disney World Resort
If told in advance they can also accommodate most food allergies etc.
I liked Mickey's Backyard BBQ... the food wasn't the best, but it was delicious. There wa a 79 yr old, two 50 yr olds, an 18 yr old, 12 yr old, and an 11 yr old. They all loved it. Goofy came out and danced with my grandmother, and there were "shows" for the kids. There was a lot of time for dancing, which can be fun for all. It was loud there, so I don't know how the baby will feel about that. It was set up like picnic tables, and they were long so there is a lot of room for large families. I think your family would enjoy this and should definitely give it a try!!
I am not a big fan of those things and even though the food wasn't Disney's best, it was still good and I had a lot of fun. (I was 18) I also lie to make the best of every situation so I made sure I was having fun while I was there!