Hi, Size000Hero,
This looks like an interesting vignette! I've got a few ideas for you, since you asked for suggestions on getting the effect you want:
There is glow in the dark paint, but I'd advice against it: You wouldn't really see the scene, only the shapes of the mushrooms in the dark. A better idea might be to use fluorescent paints on the mushrooms, and mount a UV-light LED on the ceiling of the "cave", out of sight from the viewer's perspective. This way, in low light, the mushrooms would appear to glow. You might also lay some fluorescent color around the scene nearby the mushrooms, suggesting a bit of OSL ambiance. With a resistor and a watch-battery (and maybe a toggle-switch?) stuck into the bottom of the display-housing, you'd have your luminescent mushrooms and underground scene. This would be *AMAZING* for some kind of dark elf diorama! (So cool, in fact, I might do some version of this myself...)
I'm guessing you want to finish the outside with stain and varnish, which will look gorgeous. If you're not attached to that idea, this would be much easier to fabricate out of putty and a small box.
In any case, I'm looking forward to seeing this progress. More pics, please!