The Fairy World.

This book is a kids book. This book is short but may be an enjoyment to small kids.
This is the introduction:

INTRODUCTION
Did you ever think, if a thing called "fairies", really exists? Well, Sally Andrew and her sister Marcie Andrew didn't think that it is true, until they secretly became friends with a fairy called Rosetta. But, Sally's and Marcie's mother, Mrs. Andrews priceless necklace got stolen by a gang of thieves. Where would the the fairies find it? And, more importantly, how will they explain who had found the clues since the girls had said "friends" but when their mom comes to thank them the "friends" aren't there? Everybody knows it is simple pimple to just say that Sally Andrew and Marcie Andrew found it, but they wanted them to know that fairies really are real.