The best music study at the grammar level is to learn how to sing well, on pitch. Tone-matching is easiest when a child copies the clear voice of a female teacher, rather than an instrument. The second best way for a child to learn the grammar of music is to take up the study of an instrument.
Piano is the basic choice which comes to mind.
We have found the Alfred piano series to be a good program. And by the way, there is nothing wrong with making our children practice an instrument. A child equipped with a strong study of these four elements through vocal or instrumental study would be ready for the dialectic stage of music. A final area of study appropriate to the grammar stage is music history. A good method for the study of melodic intervals at the grammar level is the Kodaly method.