Two basic texts, subtitled A Systematic Approach to Bible-Based English Grammar and Using Punctuation and Capitalization, focus on grammar basics and can be used whether or not you choose to teach diagramming. Ungraded, they can be used with all ages, although I would suggest ages 8 or 9 and up. A separate answer key is included for the first textbook.
AMG also offers a third book in this series, subtitled English Grammar Diagrams Based on the book of Joshua. This was the original book that attracted my attention. It offered the most thorough treatment of diagramming I had ever seen to that point. Assuming a foundational knowledge of parts of speech, it demonstrates simple to extremely complex sentence diagrams using Scripture verses. Students are given sentences to diagram, and "solutions" are shown at the back of the book. It serves as a companion to the other books, but it can also be used to teach diagramming on its own.