I have used GE 100% silicone, it is the silicone thast has only one ingredient, VOC. I have used to make tanks, reseal them, repair things and anyhting else I needed with no ill effects. I made a small tank for baby swords out of it and never lost any.
As for a cave system, I would use the PVC
and coat the entire thing with rock. If that is a lot more work than you want, try the pvc
canvas with the rock although stacking it would be a real pain.
Always let the silicone dry until you can't smell it. that way it should be safe.