The Weasel and the Wizard — Nan Whybark

Living in a secluded forest cottage, Krystin, age ten, starts longing to know what lies in the world beyond. Her parents, fearful of something they refuse to talk about, forbid her to wander far. When Krystin unexpectedly encounters a talking weasel in the woods, she and her younger brother, David, soon find themselves on a dangerous secretive rescue mission to an evil wizards castle that will forever change them and their world.