I probably read this children's science fiction story in the late 1960s, so the similar-sounding 1980 book, This Time of Darkness by Helen Hoover can not be it. It may have been a short story, but I am not sure that it was not a short book.
See also for discussions of This Time of Darkness:
I'll try a stab in the dark based on the few details given :) - Dark Universe by Daniel Galouye. Doesn't appear to be a children's novel but was published in 1961, is rather short (154 pages) and deals with an underground post-apocalyptic civilization. Features a boy (Jared) and girl (Della). I think the synopsis provides sufficient detail that if someone had read it, they would recognize it.
Nominated for the Hugo in 1962. Still in publication. A Google search for "Dark Universe.pdf" will turn up PDF versions.
Hmm... might be The City Under Ground by Suzanne Martel in 1963. It was the primary inspiration for the Book of Ember series that came out a few years back.
Additional Info: The city dwellers believe themselves to be the only people left on Earth, but find out that isn't the case. The main character escapes with his brother and encounters a girl from a different city who is telepathic. Anything close?
Another stab in the dark: Wine of the Dreamers (1951) by John D. MacDonald. It does feature an (alien) underground city inhabited by children or childlike beings, although the underground city is not a major plot point.
I actually remember two stories something like this, that I read in elementary school. They were in those Mifflin Readers, or whatever they were called. I think one of them was called "Galaxies" and the other was "Serendipity"?
My memory is vague, but I remember both stories involved a boy and a girl, living in a future, underground society. They were told that the outside world had long since been poisoned, and was uninhabitable. In one of the stories, it ended with the two kids coming out of a cave, and seeing a lush valley with plenty of trees and greenery, and the world had pretty much healed itself. In the other story, the two kids came to a sealed door at the top of a long ladder, or something like that. They could hear something scurrying, on the other side of the door, and were excited at the prospect of a healthy, pristine world out there waiting for them. But when they got the door opened, they discovered nothing but heaping piles of garbage, a polluted world, and the scurrying sound they heard was that of rats.
I have no idea what these stories were named. I do remember the first story, with the optimistic ending, had some illustrations that were done in a sort of trippy, psychedelic 70's cartoon fashion, but the second, more downbeat story, was illustrated in a more realistic, dark fashion.
I'd really be curious to re-read these stories again. Hopefully they're still out there, somewhere.
