I ended up improvising an adventure for my son and a friend of his last week and decided that I'd flesh out the map for the next time we play.
They entered in the lower right corner via an underground river, and are eventually going to work their way up and out into an open, ruined basement of an as-of-yet descriptionless building.
It was neat because as they were exploring all I was really doing was describing intersections and hitting them up for ideas and asking them what they saw. When we'd get to a room, I'd ask what their character saw and then we'd build off of that.
The first room had a waterfall and a couple of lizardmen, and between them (they're both 10) they decided that the lizardmen had a colony hidden through an underground tunnel under the waterfall ("but we won't be able to hold our breaths long enough to swim to it") and that the lizardmen were hiding from a monster who lived in the caves called "Thasalish."
At the first intersection I asked them what the difference was between the two tunnels and they decided that the one on the left smelled funny and that they were going to investigate, so I filled a cavern with a bunch of mushrooms, and they filled it with a bunch of little mushroom-people farmers who couldn't speak but tried warning them about going any further.
All in all it was kind of an eye opening experience. It was really amazing watching their minds work, and though I'm sure a lot of what they were feeding me was rehashed cartoon episodes and video game plots, I was still impressed.
Very cool Nytmare. I like how you've done the walls and the elevations. That's a pretty neat idea of asking your son and friend what they saw. Kinda reminds me of the movie Bedtime Stories. I might have to try that with my 8 and 5 year old.