Assuming you would want to stay on-site, both because of band activities and because it will be easier to get to/from parks, you could do two connecting rooms at any resort or a 2 or 3 room villa. There have been a few threads lately about staying at WDW with a large family. I think there were both on and off site suggestions.
Was a block of rooms booked for your son's band? If so, I would guess it would be at one of the All-Star resorts as that seems to be where most youth groups stay.
Except for a few restaurants, advance dining reservations are not yet open for the dates you are travelling. Dec 22nd is supposed to be the magic date that the changes to the dining system are finished and your dates will open up. Make sure you call early for any must-do restaurants, especially since you'll be at least a party of 7.
My suggestion for your age range is to do most meals together as well as any parades & shows that are fun for all, but allow the older kids and younger kids to have some separate activities as well. For example, go to a park together but one of you ride the thrill rides with the older kids and the other visit attractions geared toward the little ones. Even though the older ones may not be into the characters, I have found that they still enjoy seeing younger siblings/cousins meet and greet.
Biggest advice: have fun! You won't be able to see/do everything. So look at what attractions are at each theme park, what shows/parades you'd like to see and have a list in mind. Don't set a tight schedule, but have an idea of what park on what day, along with a list of dining times you are able to book and run with it. The kids can help too in the planning - might make it more fun and a lot easier for you as well.
