When each child reaches K or 1st (depending on the child and what all is going on), we begin slightly formal musical instruction with a lap harp. Lap harps work well for early music instruction because each string makes one note, so the child doesn't have to try to make the instrument produce the right sound, and the song sheets lie under the strings and show the child where to pluck. I've used three different lap harps, one for children, one for adults, and another one for children that I just bought. This last is by far my favorite for this type of instruction: Hape Early Melodies Happy Harp. During our lap harp phase, I expect to work on rhythm, music reading (just learning to read the different types of notes), and hearing the scale. I tune the instrument myself and sit with the child during each lesson and most practices.
After a year or two of lap harp, each child has moved to a recorder. I never have bought a recorder because I was given some by a friend, but ours are plastic and made for learning on (not just toys). Recorder lessons are done independently using an older version of this book: Recorder Fun! Teach Yourself the Easy Way!.This instrument does not need to be tuned, so I do not need to be involved in that way. The child does have to struggle at first to learn how to make the recorder produce the desired sound, something not experienced with the lap harp, and by the end of the book the music is written in standard notation so that the child is learning to truly read music.
After a year or two of recorder, we've had the opportunity to move on to actual band instruments taught in a band setting once a week, so I've not needed to teach any more than that.