Well, actually, we do see R.'s little blond kidlets while in the Gulch and there is that train worker boy who also ends up in the Gulch we see twice...

The term adult in both literature and age is sadly non-exact. I mean, lots of teenagers or even younger children read very "adult" books. But the way D. and F. are presented as children is more like tiny adults. Most adult books do a better job at making the children characters seem like children. Also, there's less of the Venus effect; the characters have usually developed from child to adult, even if that was before the begining of the book.