The best music theory I have ever encountered was not written down, unfortunately, but was taught orally by my daughter’s viola teacher. This grandfatherly gentleman has a background in math and physics and also music (classical, Scottish, and American).
Unfortunately, I have not yet convinced him to write his method down so for now the best music theory material I have found is actually freely available from a music theory professor at the University of Pittsburgh: Gilbert DeBenedetti. The material is available for download at “G Major Music Theory.” It works well for any child who can read at about 4th grade level, although you probably need to coach the younger ones through a bit. Not to worry, though, the material comes with answer keys and starts at such a basic level that it is easy to work through for anyone with any amount of musical knowledge.
So if you are thinking about curriculum for the next school year, and want to include music in your child’s education, G Major Music Theory is well worth a look.