You honestly find more to enjoy in "And the Children..." than "The Lights of Zetar"? You can't be serious.
"Zetar" has a beautiful, grown-up guest star with poise and stage presence and smokin' hot stems. It has a decent role for Scotty. It has vastly better music, even given that it's a re-use of WNM. It has richer, more vibrant cinematography, better depth, better lighting; I think it's the best of the Al Francis shoots.
"Zetar" has some fighting action for the Enterprise and some exciting interior scenes that are almost like "Battle in the Mutara Nebula", with the crew rushing to meet the challenge being posed by an enemy.
"Children" has annoying, non-presence guests, young and old, who don't belong on the same stage with Shatner and Nimoy. Belli in particular was not even learning to be an actor, he was just a rich lawyer appearing for the fun of it.
"Children" relies on an artificial situation in which an alien alters the minds of our heroes, so they lose both their intelligence and their courage. They conveniently have do anything the plot wants, with no motivation beyond "a wizard made them do it." Unmotivated action is a hallmark of really bad writing.
I can almost understand most of the other votes that I disagree with. But I'm aghast that anybody is voting for "Zetar" as worst of S3, when the "Children" is standing right there in all its wretchedness.