It seems like conventional wisdom says kids want to read about protagonists who are as like themselves as possible. In this country, that often (though less often now than in previous decades) translates to fewer stories that take place in other parts of the world or that have characters who look different from the majority population. As many have pointed out, it definitely translates to fewer book covers depicting anyone or anything "different," because such images might tell some readers, "this story isn't
But kids do want to read about, say, Hogwarts. And Panem. And Camp Half-Blood. I'm fairly certain none of the customers I've encountered have Greek gods for parents, but they sure are eager to read about kids who do. Obviously, I'm not equating any real place with a fantasy world. But kids' interest in reading about them indicates to me that they're very capable of imagining settings they've never seen, and many of them want to.
Of course, many books with dark faces on the covers tell very serious stories, and these stories are worth telling. So are the serious stories out there with white kids on the cover. But for every Mockingbird, there are ten or so Ramona books, so no one gets the message that these books aren't fun.
Sometimes, you're just in the mood for a fun read, and that's okay. When that happens, I may just point you to Anna Hibiscus and her hilarious cousins in Africa--amazing Africa.