The Sherlock Files, book 1 : The 100-year-old secretTracy Barrett2008

Welcome 12-year-old Xena Holmes and her brother, Xander, Sherlock's great-great-great grandchildren, newly arrived in London from Florida. They are playing a favorite game, guessing people's occupation by appearance, in front of their hotel when a strange man delivers a note written in disappearing ink directing them to the Dancing Men Pub. There, through a set of tests, their famous relative is revealed to them, and they are given his "Unsolved Cases" notebook. Xena, intrigued by it, soon finds a case with modern relevance—a missing portrait by Nigel Batheson, whose other works are being displayed in a nearby gallery. The siblings are off and running through a plot laced with references to the original stories and using similar devices, including a red herring. The main characters are observant, bright, and gifted with powers of deduction. Watson's great-great-great grandson, Andrew, is integral to the plot. Some clues seem to be obvious, but they can lead to a wrong conclusion. A well-paced beginning to a new series.