This is how I get Max to practice:
Get my bass and his violin out. Ask Max to help me learn his pieces on the bass (i.e. Suzuki book 1, ABCs of Violin, Fiddle Magic, etc…) Play the piece he’s working on and ask him to point out any mistakes. Intentionally make many mistakes. He points ’em out, with much giggling. Ask him to play it for me so I can hear it. And voilà – he’s playing it just fine. Repeat, simply making mistakes where he needs to work on it.
Bonus points: I get to work on my thumb position technique and treble clef reading at the same time.