The Spiderwick Chronicles

Helen and her children, 15-year-old fraternal twins Jared and Simon and their sister Mallory move to their ancestral home, Spiderwick. Jared discovers a boggart and realizes that magical creatures are real! The only one to believe him is his great-aunt Lucinda who implores Jared to find the pages of her father's field guide to magical creatures and protect them from the murderous Ogre, Mulgarath.