Hi,
Yes, they have the campfire program every night in Dec. unless their's incliment weather (mainly rain.) It is free and no reservations are required. Usually, alot of the folks staying at the campground use this benifit as it's the most convient for them (and they don't get Disney movies in their room.) You can also go as a Disney resort guest, but this is what you'll need to do to get there...
If you have a car-
You'll need to drive to the Campground and park at the front near the main entrance. Then you would take the campgrounds internal bus back to that area. The buses run every twenty minutes and stops at all bus stops along the way around the campground. Luckily, your destination is near the front of the resort area.
If you DON'T have a car-
You'll need to take your bus to either MK or Downtown and get on the Fort Wilderness's bus the same way as above. Or if you "do" go to MK you can take the campgrounds water taxi, but it puts you out at the back of the campground (and remember you need to head towards the front) so, again you'll need to get on the internal bus there at the campground and do the opposite as the above.
It can be done, but don't plan on trying to fit to many other things in on that night as it will be an adventure.
Have fun...
Mmketeer