Jan, I like the entrance ideas! The cave is in the room where all of our elementary children meet for worship, so we're looking at a room that is roughly 30' x 40'. So far, I plan to cover one wall with black fabric (I have that already so it's no extra expense) and use crumpled brown paper, with some gray and black spray paint, around the edges to make it look as if the cave goes back even deeper. I'd like to try and cover at least 2 of the remaining walls with paper or canvas or something to make them solid walls but I'm looking at a large surface area to try to cover. I wondered about using some refrigerator boxes (I've got a friend at ABC Warehouse who can keep me well supplied).

As for covering the ceiling, my concern is lighting, I want the room to be more dim than usual but not dark, we'll have kids as young as 2 and 3 in that room so I don't want to make it too scary. I wondered about using some kind of indirect lighting that might give it more of a fantastical feel but I've only got fuzzy ideas about how to do that (maybe a strand of colored rope light behind some paper to make some kind of glowing rock formation).

I'll probably have to scale things back from my vision to meet budget and time constraints but hopefully this gives some idea of what I'm thinking. Thanks.