What genre does a childrens book about work fall under?

This really depends on your source. If I were going to group them, I would use:

First Chapter Books: Generally things like the Magic Tree House books, for about second and third grade reading level.

Regular Chapter Books:

Fantasy: Such as Harry Potter, The Hound of Rowan, and Leven Thumps

Mystery: Such as Sammy Keyes, and Boxcar Children

Historical: Such as Blood on the River, the American Girl series, and anything by Giff.

True to Life: Such as Judy Blume, the Candy Apple Series, and Diary of a Wimpy Kid

Goofy/Comedy: Such as The Day My Butt Went Psycho, The Prince of Underwear, and Molly Moon.

Really, you could group them however you choose; some people separate out series books, such as Boxcar Children, from regular chapter books because of the shear number of books in one series.