I agree with holding off on the cleaning of the machine - it doesn't take much to lose your audience. When we had 5th & 6th grade girl scouts we did book bags. We started with them decorating with the puff paint and the next week we sewed them together. It was surprising how many of them actually used them. Pillow cases could be a similar reaction. The idea of sewing on paper would be a nice starter - but not too long, they will lose interest.
Will they all be using the same machine? Model?