Some of them may not have ever used a sewing machine, so maybe making a non-pieced pillowcase first might be a good intro project to sewing a straight seam.
My Great nieces were also influenced by the actual fabrics/colors. I had some precut stuff but if they didn't like the colors or prints, they were less engaged. The precuts are a good idea, but maybe make up some 'kits' of different colors and let them choose. Make sure you have more kits than kids so they all feel like they had a choice. MAybe draw numbers for the selection order. Or instead of kits, have precut squares of different prints and a single neutral. Each sewer could pick "X" number of printed squares + the neutral.
The 9 patch idea is a good one, but I would skip the alternating solid squares. Even though this would get their quilts done much faster, if you precut the large squares, their seams will need to be correct for the large squares to fit.
I would also think about the ratio of instructors to students. For the little ones or non sewers you are looking at a one-to-one.