Children's book I want to remember--somewhat SF

I’m trying to remember this children’s book, a sort of post-apocalyptic/1984 thingy.

Two kids, boy and girl, live in huge underground city. Lots of grimy corridors and rooms, no sky, processed food at stations sort of thing. It’s very crowded, and I think couples are only allowed 1 or 2 children. They go to ‘school,’ where they are taught to understand simple picture signs but not to read. Eventually they explore and discover that if they go up enough stairs and so on, they can get out. They discover the outdoors for the first time, and find something more delicious than pureed chicken. It turns out they’re an oppressed underclass, I think.

Can anybody help me here? Thanks!

That’s This Time of Darkness by H. M. Hoover (I’ve got a copy on my bookshelf - it was one of my favorite YA science fiction books and probably the one that got me started reading that genre):

You could probably find a new copy - Hoover’s books have been re-released in the last several years.

Yes! Thanks!