G Major Music Theory offers (among other things) free downloadable music theory workbooks suitable for high school aged students, though you may be able to use some of it with children even younger. The first three or four chapters (divided into smaller weekly lessons and then practiced and reviewed before moving on) would make a nice music curriculum for younger grades.
The music theory series, which he calls “Pathways to Harmony” is located under the “Music Fundamentals” tab. They also offer piano music and guitar music for beginners on up and other music theory practices that would be more suited to the higher grades.
G Major Music theory has everything you need to begin teaching your children about music at home.