Cut out the insides of a pumpkin (the guts).  Replace them with strips of paper with the song to sing.  Choose reverent child to pick song. 
I will also have black and orange balloons push-pinned on a board with ways to sing the chosen song:
The Bat:  Flap arms like bat wings
The Mummy:  Put both arms straight out in front of you and rock back and forth on straight legs
The Werewolf: Make claws out of your fingers and move hands from side to side (like the Thriller dance)
The Witch:  Stir a pretend cauldron
The Child:  Rub stomach like you ate too much candy
The Skeleton:  Shake your bones!!!
The Vampire:  Turn out lights, touch ear to shoulder like your neck is going to be bit, and make fangs with fingers