A Little More Holmes...

There have been a number of authors (both adult and childrens) who have written books that feature Sherlock Holmes as a character. The size of his role varies but authors love to use him because he's such a well-loved character in literature. Nancy Springer created a younger sister for Sherlock and Sherlock himself does appear in the Enola Holmes stories. Here are a couple of other books that include Sherlock Holmes:

The Case of the Baker Street Irregular: a Sherlock Holmes Story by Robert Newman (1978)

Brought to London under mysterious circumstances by his tutor, a young boy seeks the help of Sherlock Holmes when his tutor is kidnapped and he himself is threatened with the same fate.

The 100-Year-Old Secret by Tracy Barrett (2008) - Sherlock Files #1

Xena and Xander Holmes, an American brother and sister living in London for a year, discover that Sherlock Holmes was their great-great-great grandfather when they are inducted into the Society for the Preservation of Famous Detectives and given his unsolved casebook, from which they attempt to solve the case of a famous missing painting.