Great episode. This really is a terrific episode when you think about it. The episode itself is dark at times, funny and intense. The special effects were limited but Gene incorporates a lot of imagery to make the episode interesting.
Kirk and landing party find a previous expedition dead but their children are still alive? How and why this is will be explained. In life one finds out that the truth to many ugly things usually harbors a bitter secret.
In this case the children of the dead team under a bad influence do the unthinkable to their parents.
They don't know that they've aided the murder of their parents because they've been manipulated into doing so. That is the point of this episode. The manipulation and control of one authority figure over young kids.
Groups like Third Reich did it to young kids in Germany. You find more relative examples in something like a gang where one mature gang takes advantage of a younger one. You get the picture.
This makes the finale of the end that more memorable. Kirk decides not to use force to drive the entity away from the ship but make the kids realize that they've been manipulated the whole time to do some truly evil things.
That's a lesson that most parents and educators should really take not upon. That is a resolution not by force but driven more by understanding.
This is an excellent episode with a lot of interesting themes to it. Sure it's a bit funny at times with the whole emphasis of the kids taking over the ship. However, once you really piece together what the episode is trying to say you realize it's a great episode.