The Tales of Beedle the Bard contains 5 short fables, purportedly translated by Hermoine Granger, with commentaries by Albus Dumbledore and additional notes by J. K. Rowling. I think it is so clever and fun that Ms. Rowling creates books that come from the fictional world of Harry Potter. She is following in the steps of great myth makers like Tolkien and Lewis in creating an entire world, and though I wouldn't go so far as to put Hogwarts on the same level as Middle Earth or Narnia, I do think these books will have lasting value and appeal. In fact, I would classify them as new classics, mostly because Book 7 wraps up the entire series with such a clear redemption narrative.
But back to Beedle the Bard. . . the fables of the wizarding world are not so different than Aesop's fables or other such tales that convey a fairly simple story with a moral lesson, only instead of drawing on nature, these stories tell of magic used for good or ill. They teach the value of humility, the folly of selfishness, the worth of all people, consideration and care for others, and the finality of death. I think they would be excellent to read aloud to children as young as 5 or 6, for there are certainly parallels that could be drawn between these tales and biblical truths and values, giving a family plenty to discuss even before reading the Harry Potter series.
For those who are still skeptical about Christians reading and enjoying tales of witchcraft, I would recommend Looking for God in Harry Potter. The The Tales of Beedle the Bard will help you to find the underlying moral values of the wizarding world Rowling created, and Looking for God in Harry Potter will further guide you through more significant symbolism and parallels in the larger series so that you can have fruitful discussions with your children about faith, fantasy, and myth.