Coming-of-Age, Children's Literature, Family Drama
Okay, so if we're being honest (which we always are), The Secret Garden is sort of a coming-of-age story in the genre of children's literature. After all, both Mary and Colin change and develop over the course of the story, but they actually become less grown-up and more childlike and happy over the course of the book. They become versions of what Frances Hodgson Burnett seems to view as real children; they don't grow into adults like the classic coming-of-age narrative would imply.
Similarly, the term children's literature is ambiguous as applied to this novel. The Secret Garden was first published in episodes in a magazine for adults before being published as a novel for both kids and adults in 1911. While The Secret Garden is now routinely shelved in the kids' section of bookstores and libraries, it didn't used to be, which may explain the surprisingly dark and sad implications of this story of neglected and lonely children.
The one genre that we can apply clearly and unambiguously is family drama. With the abandonment of Colin Craven by his father, and the dire neglect of Mary by her parents, there are a lot of family issues in this book that either need to be resolved (in the case of Colin and Archibald) or left behind (in the case of Mary and the deceased Lennoxes). Family drama is the primary thing that makes both Mary and Colin so unpleasant; for more specific thoughts on these domestic dramas, check out our "Characters" section.