Left hand contact points

Whenever Sara plays, her left hand needs to be set in place before she begins moving her bow at all...there are two contact points;
1-on the left hand index finger between the first and second joints
2-on the left hand thumb, not on the tip but above the third joint
In between these two contact points, there should be a space, or as shown in this picture, a "worm hole."

It might be fun to have Sara paint a pencil green or put googly eyes on the pencil to make a worm or a snake.  Then, any time she is playing, Mom can let the worm/snake crawl up and check on the hole.  It should always be present, otherwise the left hand is collapsing and playing in tune will be very difficult.