When shaping the student's left hand, how can we help him or her form a "tunnel" between the fingers and the top of the fingerboard? There are several possibilities.
If the student's hand is large enough to accommodate this, I place a roll of life savers lengthwise along the D and A strings and ask the student to curve the fingers around the roll for a count of 20 or longer. If the student's hand is a little smaller, breath mint rolls may be tinier. I have also used a large or thicker "children-sized" pencil, sometimes referred to as a "Kindergarten pencil."
For young children, I make a caterpillar out of a colorful pipe cleaner. I take the pipe cleaner and wrap it like a coil in a spiral fashion around a pencil. I remove the coil and tuck the sharp ends inside the coil so the points do not stick a student. Then I use a black marker to draw two eyes on one end (or you can glue two craft eyes instead). The parent and I use this caterpillar for all sorts of things besides checking for "tunnel" fingers. We also use it to see if the student has left a "wormhole" between the neck and the left hand web between the thumb and index finger. We wiggle the caterpillar into the little hole.
There are many ways to accomplish this. Look around and see what might be useful.