To sign between, take your non-dominant, open, flat hand, with thumb pointing up, and hold it steady. Your open, flat, dominant hand will first touch its palm to the thumb of your non-dominant hand, then flip back and forth as the back of its fingers then rest alternately on the index finger side of your non-dominant hand. It is as if you are choosing between your thumb and your index finger.
Between is signed just a bit like share, but with the latter sign, the pinkie side of your dominant hand is fully touching the index side of your non-dominant hand as the dominant hand slides back and forth.
Teach your child the between sign in the context of choosing between two options, or when you are pointing out that something or somebody is situated between other things or persons.
Teach your child this sign in context, when you are describing the relative position of things or persons.