The sing along is free. It is located by the Meadows Trading Post. It begins with a person on stage singing mostly Disney Songs. They may call bday people up, they may not. It does actually have 2 large fire pits for roasting hotdogs/marshmellows. They sell the makings for smores, but it is much cheaper if you bring your own. (Don't forget the roasting sticks) After the songs, they show a cartoon and then a Disney Movie. It depends on the time of the year as to what time it starts. When you check in, they give you all that info plus the dates and movies playing. If you are not staying at Ft Wilderness, the movie listings are posted by all the comfort stations and the trading posts. It is one of the must do's for my kids.
P.S. You may want to bring a blanket so any little ones can sit in front of the movie screen, bug spray and your own chairs as the benches and bleachers seem to get uncomfortable after awhile.