The Secrets of Ordinary Farm. by Tad Williams, Deborah Beale (Ordinary Farm Adventures)

Tyler and Lucinda have returned to a far-from-ordinary place for the summer. Instead of pigs and sheep, this farm is home to dragons, unicorns, flying monkeys... This summer, everything has changed - security fences are everywhere and Ordinary Farm is in no small danger. Shortly after Uncle Gideon declares he wants to leave the farm to Tyler and Lucinda, he goes missing, and the children suspect the evil hand of the witch-housekeeper, Mrs. Needle. But then Gideon just as mysteriously returns, weak and unwell. Mrs. Needle persuades him that the children are conspiring against him and, suddenly, horribly, the tables are turned... So begins an adventure to save the farm, involving a magnificent dragon, magic mirrors and a haunted mine.