Step 1: Nag them to practice their violins.
Step 2: When they ignore you and play around on the piano instead to procrastinate, let them.
Step 3: Don't try to teach them at all (because then they won't want to do it).
And, as Miles would say, "VOILA!"
They've been experimenting (completely by ear) with how to make their 2 hands sound good together. I like what they've come up with so far.
Miles' adaptations of a couple of his violin songs:
Colsen's original composition (edited for length):