I'll try a stab in the dark based on the few details given :) - Dark UniverseDark Universe by Daniel GalouyeDaniel 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.