A co-op preschool, similar to the one mentioned in this NYT piece here might work for you. However, a lot of co-op preschools require more volunteer hours on a regular basis from the parents during business day hours, to keep things running.
The key to a successful preschool (IMO, and studies are showing this to be true) is that play IS learning. Play is the work kids do. So, having a qualified teacher is the best thing (one that can help with their emotional/social development and help them navigate social skills, verbal skills, etc.). Having structured playdates at least weekly or bi-weekly where the play is guided (and having an understanding of what works in helping them learn) can go a long way toward this; part of the playdates could structured (in good weather, going on a scavenger hunt in the woods, with kids working as teams or whatever), and part of the playdate be free play (free play is HUGELY important, imo) with parents to guide the kids as needed.
If it were me, I'd probably check out the underground preschools, knowing that it could be a sketchy situation. I'd make sure that if I did go with an underground one, the teacher WAS background checked (it's not that big a deal to get it done) and if at all possible, certified in some fashion. If I didn't find one I liked, I'd go with the scheduled playdates with a group of kids (4 or 5 max), even though it would make my life harder. And if I did the playdate thing, I'd start taking some parenting courses or ask friends who do preschool for some tips/help in doing preschool stuff at home -- they would be a wealth of info.