For the sake of the argument, I'm going to assume you meant that these kids were hypothetically really far back into the cave where it's pitch black, with absolutely no visual stimuli at all...
It is my understanding that neurons to an extent are a use-it-or-lose-it type thing. You're born with a huge set of possible connections, but only the ones that are reinforced remain after a given time, and the rest weaken. (Strong argument to introduce your baby to many varied situations/stimuli, perhaps?

) If your visual pathways never were used from birth, the lack of stimulus during critical formation could conceivably render you permanently unable to see.
Nevertheless, neurons also exhibit great plasticity, so who knows what would happen if you took them outside? Probably depends on the age at which you did it... At 40 y/o it'd probably be too late, but if you took them outside as a 4 y/o perhaps they could adapt? Pure speculation on my part, though...

There might be relevant studies with various kinds of stroke patients out there?
As some scientific back-up to my speculation, there have been experiments restricting kittens' eyesight from birth (since they're born blind), with either dark-rearing, or restricting their visual field in various ways to see the permanent effect. The evidence suggests some preference for whatever they were exposed to...
Couple relevant articles:
http://www.ncbi.nlm.nih.gov/pubmed/681993
http://www.ncbi.nlm.nih.gov/pubmed/7230034
http://www.ncbi.nlm.nih.gov/pubmed/18019611
Enjoy!
