The aim of having a World Book Day is simply
"...to encourage children to explore the pleasures of books and reading by providing them with the opportunity to have a book of their own".
Read more about it here . There are lots of tokens and freebies and everything else a little kiddie would enjoy, e.g. books going for £1.00.... a site where kiddies may upload their stories! Wow!
So get them young, is wot I say! Start the little dears reading and writing stories and using their libraries and being generally creative and excited about the written word whether online, e-print, whatever!
So grab a kid and make them read!