on Indies Unlimited:
The genre lists are basically an oleo of labels, developed organically over the years, meant to be marketing tools, and their primary objective is to give prospective buyers a hint as to the kind of story they are likely to find.
But Children’s, Young Adult and to some extent New Adult are meta-classifications based on the age of the reader.
As a sub-heading under that, you can list all the usual genres: Children’s Science Fiction, YA Romance, etc.
So when writers are choosing their genres, they can’t deal with the age genres the same way as they treat the regular ones.
For example, the age guidelines were created for three separate reasons.