Hmm. As I think about it more, I'm backing away from my brain-damaged theory. (Hey, that works both ways: it could mean that it's a theory about damage to brains, or that the theory itself is brain-damaged.) There are too many ways, some of which you've mentioned, that the kids appear normal. For one, their art would have never been displayed if it was terrible.
I guess the kids' passivity can be blamed on the isolation and control of their upbringing, plus the utter indifference the outside world shows to their fate.

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