Arthur and the Invisibles (2006)

Ten-year-old Arthur must find a way to keep the home of his beloved grandmother from being destroyed by a developer with other plans. Remembering his grandfather's story of a treasure, he enters the world of tiny beings who live in perfect harmony with nature. Together with Princess Selenia and her young brother, Betameche, Arthur goes in search of the treasure and -- along the way -- discovers that his long-absent grandfather may not be missing after all.