Here is another third season instalment that is considered a very bad episode by many. Personally, while it is far from 'Star Trek's finest hour, I don't find it to be out-and-out terrible. It's certainly better than a certain "Spock's Brain" a few episodes ago.
The story has elements of the first season's "Miri", both concerning children who harbour a sinister secret.
I don't know why the story is held with such low regard amongst many fans, as personally I think it has a semi-decent plot. I think the main flaw of the episode is that it has a reasonable concept, but doesn't unfold all that well; the story isn't developed half as well as it might have been.
I agree with another reviewer that maybe they weren't able to make it as chilling as it might have been, with the limitations of 1960s television requirements; the story is crying out to be much darker.
The effects that the various crew members suffer under the influence of the children is a mixed bag; Uhura seeing a terrible 'reflection' of herself is good, but I wasn't as convinced by Sulu hallucinating about piloting the Enterprise through the giant 'space daggers' (!).
Kirk's breakdown is nowhere near as good as various influences in the first season episodes "The Enemy Within" or "This Side of Paradise"; and his sudden 'recovery' seemed rushed and forced.
It's also hard to decipher what exactly the 'point' of the story is, as in what is the moral. There seems to be elements of stranger danger, vague child abuse and all sorts mixed in, but the 'point' is not very clear.
[Minor spoilers] The climax with Gorgan is an interesting one, as it is not a fight or a battle, but a simple case of Kirk convincing the children to 'stand up to him', thus robbing Gorgan of his power over them.
The final moments of the episode, as the children finally start to cry, is probably the best moment of the story, and makes for a good closing. [End of minor spoilers]
All-in-all, not as terrible as some would make out (as I say, I consider "Spock's Brain" to be far worse), but hardly a great episode either.