Include the back matter as part of your manuscript and call it out in your query letter. That supporting material is a component of your project, which should be considered in full. There’s no point in telling the editor about potential back matter but then leaving her to guess whether or not you could create truly interesting and useful material. If the editor thinks the back matter isn’t necessary, she’ll focus on the text; you won’t have hurt yourself by including it. Just remember that back matter should expand the book’s usefulness or interest. Adding instructions for building a kite simply because a kite appears in your story may seem fun, but it’s probably unnecessary. If the kite figures into the story in a crucial way that implies buying or building a kite is important, then it makes sense to add material about kite building and flying. Nonfiction picture books are more natural candidates for back matter, but fiction can benefit from it, too.