April I saw this on Twitter the other day which has some lovely-looking book recommendations for kids, some of which would be fine for kids under 14. mikesmathpage.wordpress.com/2016/01/18/some-book-suggestions-for-a-14-year-old-who-loves-math/
The Number Devil by Hans Magnus Enzensberger is a primary child-friendly introduction to things like infinity, permutations, imaginary numbers.
For older secondary children Simon Singh's books on code breaking, Fermat's Last Theorem and the maths in the Simpsons are pretty accessible although written for adults, and Marcus du Sautoy is also very good.
Actually, the book that made me decide to study maths at uni was Jurassic Park by Michael Crichton. Ian Malcolm (Jeff Goldblum in the film) was the hero! It talks about fractals and chaos theory.
My favourite book about maths is a fictional novel, Uncle Petros and Goldbach's Conjecture by Apostolos Doxiadis. It goes into number theory, proof and the devastating blow dealt to maths by Goedel.