The "Miri" children can be annoying, but I'd rate this episode fairly high. For one thing, it's one of the episodes in which you really get the feeling that the problem has to be solved by just doggedly working it out. The show could have been written with, say, a resolution in which Miri tells Kirk that there's a secret tunnel the kids avoid because they associate it with the grups; she leads him there, and wheee! there they find the antidote that the adults had figured out but hadn't been able to manufacture in time....or something like that.
The "alternate earth" element is problematic. Watching it now as an adult, it seems obvious that a big reason was the convenience and cost savings of being able to use existing studio lots. One could also say that the planet's victims have to be very like earthpeople in order for the disease to transfer so readily to the Enterprise crew. But it's unsatisfactory to have these "extraterrestirals" live on an alternate earth with a wave of the hand. On the other hand, if the script had had it that the victims are descendants of a forgotten earth colony, one could object that the ruins shouldn't look like late-19th-century/early-20th-century structures.
Bottom line is that it does work, or did work, for some viewers, like me, for the first viewings. I'd give it a "B" and if I were winnowing down the series into, say, 25 keepers, would expect this one to make the cut.