My hs group has co-op day once a week. We offer 2 class sessions for *school-age* kids (2 course options at a time) and a pre-school program. Then we break for lunch and playtime after which most folks stay to participate on one of 3 Odyssey of the Mind teams (pre-schoolers play or watch a video in a "quiet" room).
Things we make sure to do:
1. Keep class sessions short, we only go 4 or 5 weeks before offering a new crop of courses. Good taste of something for the kids, nobody gets bored, makes it easier for adults to commit to teaching/facilitating.
2. Speak up if there is a problem, but be ready to be part of the solution.
3. We don't allow anyone to be dropped off in the care of "the co-op." Kids whose adults can't make it one day must be in the care of a specific willing adult.
4. Build into the schedule opportunities to thank the church that hosts us (for free!). We really try not to take the space for granted.
All this may or may not be helpful for a strictly pre-school co-op (I think that's what I read in the OP). One small difficulty we had to navigate w/regard to pre-school was parent expectations. There are folks who expected the pre-s. to be working on early phonics/letter recognition, there are folks who wanted a product of some kind to result from the class, there are folks who didn't want the kids to *have* to participate in any given activity... It took cooperation and creativity to work out a curriculum that fit all of our families.
Hope something here helps
