When kids are taught to throw, often the instruction is watered down into just a couple of steps. The act of throwing a baseball is not that simple. Throwing requires the entire body to work together in order to throw the ball accurately and to put something on it. All positions on the field require the ability to throw the ball accurately. Good throwing mechanics will enable you to make plays. When you warm up with the team before practice or play catch in the back yard, make sure you work on your mechanics and strive to improve your accuracy.
The best way to grip the ball is across the seams as pictured to the right. The fingers are placed over the top of the seams to provide a good grip on the ball. In the first picture you'll notice that you can see 2 seams running horizontally. The back of the ball not visible will also have 2 seams running horizontally. By gripping the ball in this fashion, those 4 seams will help to keep the ball in the air longer and keep the ball traveling straighter (assuming the player can throw it with 12-6 rotation).
It takes years of practice to be able to grip the ball across the 4 seams in this fashion when playing a position other than pitcher on the field. Players can work on this by throwing the ball into their glove and as they pull the ball out shift the ball to the correct grip. This takes practice and I wouldn't worry about it for younger players.
Try to keep the ball out on the fingertips not back in your hand. The second picture on the right shows the ball out on the fingertips. Gripping the ball in the palm of your hand and not out on your fingers will cost you velocity and accuracy. Younger players may need to grip the ball with three fingers instead of two, but unless their hands are very small they should still try to grip the ball out on the fingers.
Young players will not be able to grip the ball across the seams while playing in the field. Work with your pitchers on getting the correct grip but have your other players work on other parts of their throwing mechanics. They can work on this skill when they get older. Do work with them on getting the ball out on the fingertips. This can be difficult with small hands, but it is still an important concept for them to understand.
Many young players don't use their wrist much when throwing the ball. When the ball is brought back in the throwing motion, the wrist should be cocked back. This way the wrist can be used as part of the throwing motion.
Watch young players throw and you will see most will throw with a stiff wrist. It is very difficult to throw the ball accurately with a stiff throwing wrist. This is a skill that young players should work on from the start.
You can practice this skill by holding your throwing arm just above the wrist with your glove hand (see image to the left). Bend your throwing arm at the elbow with your forearm vertical. Keeping your arm in this position, practice throwing the ball with just your wrist and fingers. It may feel strange at first, but keep working on this skill. The wrist and fingers play a major role in the accuracy and strength of your throw.
You can find more throwing drills in the drills section.