Some first genres to consider:
ABC or numbers book (or the equivalent). These allow for lots of creativity while providing a structure for the text. This can reduce intimidation and fear around writing.

Family book. Often this is a book for a child or grandchild and it explains the family. It's less structured in format but it relies on information that does not need to be looked up, for the most part.

Parody. A recent non-children's book, "Go the to Sleep" is a parody for adults on children's bedtime books. Parody provides a topic and even a language to use. For example, the adult parody includes the kind of language one expects in a bedtime story such as a soothing poetic rhythm. In fact, the humor arises from the difference between bedtime story language and the one line in each verse that represents a hassled, frustrated parent's perspective.

Notes on writing

Plan a series of books. That way you don't get tied up into making a single book totally perfect. Perfectionism is good to a point and then it stymies creativity. If you plan more than one book, then that perfectionism gets busted up a little.

For the first series, choose something you know well--a fictional setting that is very familiar, information that you don't have to look up.

Use the real writer's Writing Process, which means not having to worry about any quality issues until you get to polishing up surface features.

Write books to solve teaching problems--if the students are having a problem with some aspect of music class, write a story about a character that struggles with this or writing a book in which students participate in the reading such that they get to practice that skill in the process of interacting with the book.