Stand on the sideline of your child's next game and listen.  Listen to the coaches.  More importantly, listen to the parents.  I am willing to bet you'll hear "Run!  Run!"  "Get to the ball!"  "Go!  Go!"  Unfortunately all of the well intentioned encouragement your voice can muster will do little to make your player faster.

Speed drills.  Plyometrics.  Weight training.  Form.  Efficiency.  Specialized, private (expensive) training.  What is the best way to improve your young athlete's speed?  How do you get your child to run faster?

Well, given nearly 75% of all young athletes in every sport suffer from heel or knee pain its a safe bet that is what is slowing your athlete down.   The primary sources of their pain are conditions such as Sever's Disease and Osgood Schlatter's Disease.  Both are directly related to the processes involved in the body's natural growth and maturation.  Playing sports aggravates these conditions.  Yes, the pain will go away if the child takes time off from playing.  However, it will likely return once they return to play.  It is safe to say that the pain they are feeling is having a significant impact on their ability to run, let alone run fast.

If you really want to improve your child's speed begin by making certain they are playing pain free.  Outside of an injury, pain from growth related conditions will definitely affect your athlete's performance.  By eliminating this discomfort you will have the greatest potential for immediately improving the athlete's speed. 
Pretty simple really.  Less pain.  More speed.

Remember, your son or daughter may not tell you their feet and knees hurt if you ask them.  They are afraid you'll either tell them they need to take time off or tell them to "suck it up" and play through it.  They just want to keep playing the game they love. 

Play hard.  Never stop.