Are there any interesting books on math for children? Let's break this into two questions: interesting books on math for children in elementary school and interesting books on math for children in middle school.
The only book I can think of that might work for kids is The Little Schemer by Friedman and Felleisen. To me, that seems almost as much a math book as a programming book.
And there's also Alice’s Adventures in Wonderland and Through the Looking Glass, by Lewis Carroll, which, although they don't really have any explicit math content in them, do have the sort of contradictory, paradoxical type of humor that many math people like.
Also Raymond Smullyan's books, but I think those might appeal mainly to older teens, but I'm not really sure.