I think that with that kind of crowd I would not try to do anything with casting unless they can guarantee you a sufficient clear space for safe casting in both directions. That would mean a roped off area and maybe a couple of adults to keep it clear. Larger flies like streamers or foam bugs will be easier to see than small dry flies. Plan on each session lasting about ten minutes less than you have per group. That will give you time to talk to those who want to know more and take a quick break