Spiking the volleyball may be the most dramatic part of the game, but the player that sets the ball is often considered the most integral player on the team.
Setting the volleyball with a soft touch is difficult to teach and requires a good deal of player skill and practice for it to happen consistently. It's important for young players to learn to keep the ball only on the fingertips and keep it moving. Never let the ball rest on any part of the palm or any part of the hand.
Your child can easily practice basic setting at home with just the volleyball and one other person. Have your child practice making a triangle, barely touching the tips of thumbs and forefingers slightly above the forehead. Next have them cup their hands lightly around the volleyball above the head so they have a feel for what it's like if they don't set it properly.
As the volleyball is passed, have them practice repeatedly moving their feet quickly to get right beneath the ball with that basic triangle above the forehead. Quick foot-work is required to move to the ball before setting it. Once in position under the ball, the feet should be shoulder-width apart or slightly wider.
For the set itself, have them push upwards with fingertips, hands and arms, releasing the ball with everything in a straight line. Think of the entire body as a spring. Using both legs means more extension of the ball from the set position. Make sure young players remember to use all of their fingers, to not only stabilize the ball but also provide more control and precise direction toward the hitters.