They start the sing-a-long right around dusk & the movie starts when it is dark. Don't bother with the blanket. Everything is on small stones. If you have your own facilities at the campground bring a folding chair, otherwise, you can sit on the individual benches which have backs. There are also bleachers, however, no backs on these & it can get uncomfortable, so get there early if you want a back on your seat.
The campfire program is a lot of fun. We do it every time we are in Disney. It is free & a nice relaxing evening for both the kids & adults. You can also bring your own refreshments if you don't want to go to the snack bar.
I love these kinds of programs that Disney offers.