Hi Holly,
My family LOVES Mickey's Backyard BBQ & Hoop-Dee-Doo Musical Revue. Typically, December is a nice month to visit Disney World. The average high temperature during the month is 72.9 and the average low temperature: 51.3. Having said that I visited early last December and it was 25 degrees one morning - unusual but it happened.
People are usually moving around and dancing a lot at Mickey's Backyard BBQ which tends to keep you warm. I'd bring a jacket just in case. I always check the 10-day forecast at
www.weather.com just before all of my trips to see if any unusual weather patterns are developing. If you see that super cold temperatures are predicted, you could cancel your reservations. Just keep in mind that a cancellation policy applies and may result in charges if you cancel with insufficient notice.
If you want to be sure that you'll have one of the two booked and guaranteed temperatures (inside), then I'd book the Hoop-Dee-Doo Musical Revue for my December trip and wait until another visit to do Mickey's Backyard BBQ.
Wishing you many magical memories :-) Allison