Arthur and the Invisibles (2007)

Like any kid his age, Arthur is fascinated by the bedtime stories his Grandmother tells him. His dreams are full of African tribes and incredible inventions recounted from an old magic book that belonged to his Grandfather, who disappeared mysteriously four years ago. When he takes a closer look at the book, Arthur realises his grandfather left numerous clues to hidden treasure in the back garden of their house. Even more amazingly, there is, apparently, a whole invisible world underground - a world inhabited by miniscule creatures too small for the naked eye called the Invisibles.

Official website: http://www.arthur-movie.com/