I loved this movie when I watched it at age 13 and again at 27 with my daughter, age 6 1/2. The story is based on actual events that did involve Sir Arthur Conan Doyle and Harry Houdini, which definitely adds a bit of history to the movie. It can present an excellent opportunity to discuss hardship as well. The family in the movie has lost a son and the cousin's father is MIA during WWI. The only warning is that Harry Houdini does say the word "ass" in one scene. My daughter understands what we call "grown up" words and simply giggled when the word was said, so it wasn't a big deal, but some parents may not want their children to hear the word in a film shown to their children.
It is definitely not a movie for anyone under 6. The concepts and actions in the film are difficult to understand at a younger age.