Chicken wire, spray insulating foam, and then paint.

I don't think bumping it would hurt it at all.  The stuff is very durable.

I would think unless you REALLY know and understand concrete, you run the risk of your walls cracking (which might be okay with it being a cave and all), but structured chicken wire formations covered with Fiberglas (as in a boat hull) would be more flexible and giving to "bumps and bruises" while maintaining its intended shape.  It, like concrete, can be painted to resemble cave walls.

I agree fibreglas to be the best solution. It's lightweight, flexible, molds easily and takes paint well. Unfortunately, it requires skilled technicians, is messy to apply and rather toxic while airborne. Proper precautions would need to be taken.

Concrete, in comparison, is quite heavy, inflexible and prone to crack. It can be painted or stained, but it is laborious. Likewise, the texture of the concrete, depending on the size of the aggregate you'll be using, won't convey the "carved rock" feeling of a batcave, but more of a "shored up mine" with a multitude of tiny rocks and pebbles imbedded within it. The surface will be rough and it's not something I'd like my dog or kids bumping into -- even accidentally. I'm also thinking of the re-sale value of your property should you need to move. Having a room filled floor-to-ceiling with concrete might be problematical.

Another option is latex-based, heavy-duty drywall mud. It can be applied over a chicken wire frame and -- although not as flexible as fibreglas -- it is not as rigid as concrete and is easier to apply than both. The great advantage is -- while wet -- it can be sculpted into the exact details you want and -- when dry -- can be sanded to refine your look even more. Treated properly, it should hold up to minor abuses.

A final option is sheets of brown craft paper and wallpaper sizing. This is the lightest and most easily applied -- no special tools required. It is a technique readily used in film & television production. Look at the caves in "Hogan's Heroes" or "Gilligan's Island" to get an idea. Of course, it won't withstand much abuse and so I'd recommend limiting the paper treatment to the upper walls and ceiling while doing an industrial wainscoting half-way up the wall (like brushed aluminum, sheet metal or subway tile.) This should keep curious hands and wet doggy noses at a safe distance.

Good luck!

Have a look at rock molded panels;-