I learned this from my saxophone teacher, back in the days when I suddenly decided that I wanted to learn to play the saxophone.
The first step is, or course, to teach the scale, on what ever instrument you happen to be teaching at the time.
Once the student is reasonably secure, you get them to improvise using the notes of the scale, while you play the progression I VI II V as an accompaniment. If you want to include 7th notes, go ahead. Why should they have all the fun?
Ah, now, to understand I VI II V, you need to understand chords and basic harmony. If you do, then this post is finished as far as you are concerned. If not, then do not panic; I’m about to post on that very topic as soon as I have hit “publish” on this one and it is all much easier than it looks!
I used this as one of the activities on a recent composition/improvisation workshop I was involved in for children aged 8 to 11 years. I picked the key of G major, hoping that it would work for the mad mixture of instruments (violin, cello, flute, cornet, trumpet, keyboard, saxophone) and diverse experience and level of play (from “I started just started three months ago” to “I’m working towards grade 4”). It worked surprisingly well, all things considered.