Here in the United Kingdom, this episode was famously absent from the BBC's (many) runs of the series for many years, due to it's content being deemed unsuitable for its timeslot. It was not broadcast on terrestrial television until it finally appeared in the early 1990s.
Anyway, this is another good first season episode of the classic series. It does indeed have its dark and chilling moments, such as the zombie-like diseased older children, and the scene of Captain Kirk being battered and beaten by the horde of children as a young girl watches and smiles.
One thing that is never fully addressed is the whole location in the first place – a duplicate of early 1960s Earth. Although the whole teaser revolves around it, by the time the landing party beam down, the plot about the only survivors being children takes over, and by the latter acts the whole duplicate Earth element is practically forgotten; it is never really explained.
Kim Darby gives a good performance of the young Miri of the title, although at times, just how close Kirk gets to her does feel slightly uncomfortable in my opinion. But even so, they make for some nice scenes.
Michael J. Pollard, who plays Jahn, the 'leader' of the children, looks a bit too old compared to the rest of them, but gives a fair performance. Also mixed in with the youths are Gene Roddenberry's daughter, William Shatner's daughter, and Grace Lee Whitney's two sons.
Kirk very much saves the day by giving the children one of his famous 'Kirk speeches'. (These even continued into 'T.J. Hooker' in 1982!). If all else fails, get Kirk to make a speech!!
All-in-all, this is a good episode. However, I do feel that there is something lacking to make it a true series classic. But it still stands as a good one.
"No more blah blah blah" indeed!