For a longer nonfiction piece you often write a proposal as part of your query, but with a picture book you'd likely have finished the manuscript draft, so you'd be querying a completed project.
So, start with a hook for interest, your pitch of the manuscript, a short bio of yourself, and mention but don't list any other manuscripts you have available (In addition to insert fabulous title, I have two additional completed nonfiction picture book manuscripts with STEM topics).
You might want to Google Picture Book Party, run by the fabulous Mindy. She runs a yearly contest and winners' queries are posted.
Good luck!