Dealing with the ball in the air poses a lot of problems for children. The first concern is when to intentionally expose them to the situation. While some kids will be able to deal with it earlier then others, takes you to YouTube, as a general rule don’t bother with it until they are at least seven. (That’s why we don’t have throw ins.) Their time is better spent developing their skills with the ball on the ground. (Like the player’s choice.) Then introduce the ball in the air in simple, non-threatening ways.

1. Simplify the service. No matter if you want to practice headers, volleys or ball control getting a proper service is a problem. Usually the coach ends up throwing the ball to each player while the rest of the players stand, wait and watch. Using a pendulum trainer as an active rest station is one solution. Another way to modify the service is to slam the ball on the ground. This gives the player a moment to watch the ball in flight as it rises and then falls. (Watch the video below how easy they make it look after ball has bounced.)
2. Change the ball. Use a lighter or softer ball at practice. A volleyball, volleyball trainer, (Excellent!) or the Brazilian rubber ball can make learning easier. Tape a string between two chairs and play soccer tennis indoors with a balloon. See the video below for the Dutch National teams example of soccer tennis. 
   
    

Practice dealing with the ball in the air in a holistic way as much as possible. Make it a game with a result. When you focus too long on an individual technique, this is especially the case with the ball in the air, kids get bored. Keep the focus on the competition, for example play four square in place of juggling.

There will be times when you will isolate a specific component, functional training, but this should be reserved for individual players as much as possible. If one person can’t take a side volley the entire team doesn’t need to practice that technique.

Basic mechanics when the ball is in the air

1. Get into the line of flight of the ball; The approach. The ball is a moving target and it may not cooperate with your positioning.
2. Choose what you want to do. This decision determines what surface you will use, where and when you need to contact the ball. It has to be made before contact.
3. The contact. Contact below the midline of the ball makes the ball go up, above keeps the ball down.
4. Heading. Keep the eye’s open and the mouth closed. Contact on the forehead. Look at the ball on the approach and finish looking at where you want the ball to go. Attack the ball and head it, don’t let it play you. (Note: There is no such thing as a defensive header. If you’re first to the ball, by definition you’re an attacker. All heading is an attacking skill no matter what Sol says!)
5. Volleys. Lean away from the ball as it approaches. The kicking leg needs to be free to get closer to parrallel with the ground. Marco van Basten demonstrates, takes you to YouTube.
6. Timing. Dealing with the ball in the air demonstrates the principle of TIC as opposed to thinking of just technique. All of the guidelines for proper technique are useless if you get there to early or to late. The correct timing depends on what the player wants to accomplish. That objective is controlled by factors like the players position, the speed, distance and angle of approach of the ball and the position of other players. It is never a simple matter of "Hit the ball here" and all your problems go away.

You have to include the context of the situation. For example, if you want a player to practice heading at goal and you give him or her a serve around their knees do they try to volley the ball or return it for a "Better" serve? Are they working on heading or on scoring from the air? What would they do in an actual game? Denis Law was one of the greats at making these decisions, takes you to YouTube. He worked with the hand he was dealt and came up a winner.

Heading drill. A three player set up. Takes you to YouTube.
Training with Zidane - Headers. Takes you to YouTube.
Heading Tennis with the Houston Dynamo - Houston Chronicle.
Attacking Heading with Sol Campbell. Takes you to UEFA.com.
Defensive Heading with Sol Campbell. Tkaes you to UEFA.com.
Soccer tennis with DC United, takes you to YouTube.
Two kids, a ball and a tennis court. Takes you to YouTube.
Barcelona beach tennis. takes you to YouTube.