Witch's Wishes, by Vivian Vande Velde, 2003

Well-intentioned wish granting goes awry on Halloween night -- and so does the attempted reversal -- when a witch rewards a kind young girl by making her costume magic wand turn real. Whether Sarah's wishes come true or turn upside-down, the results are disastrous, just as the witch's wise-cracking broomstick warned her. A short and lively early grade light fantasy novel, full of whimsy and a surprising level of complexity for the length and readership age. Strongly recommended.