I didn't post this, but after a student learns to balance the "float" and the "plop", I will often ask him/her to try to balance the thumb and fingers of the left hand in the same way. It's the fastest, easiest way I've found to loosen up the thumb when putting a finger down makes the thumb take a death grip on the neck of the violin.

With the little kids, they often don't have problems with balancing their arm (yet) because their arms are pretty short in relation to the rest of their bodies, and because they're used to tossing the violin into place from underneath anyway. But they often have a tight thumb because they're trying so hard to get the finger down, and the fingers aren't strong yet. The easiest way to loosen up a left thumb is to talk about how the thumb is on one side of a seesaw and the fingers are on the other, then ask if the thumb and fingers can balance each other so that they're equally loose. Even with weak fingers this is possible, and the concept is simple enough that the kids get it right away.