Today, if you want your children to be champion athletes, they need to start training at a young age and play their sport 12 months a year. So naturally you want to know if heavy athletic training before and during the period of rapid growth in children will harm then in any way. The most common concern is whether heavy training will make them shorter as adults or damage the growth plates in their bones. Researchers at the University of Saskatchewan studied young gymnasts, swimmers and tennis players and showed that participation in these sports does not shorten their adult height. Furthermore it did not affect the age when they entered puberty. Journal reference
The biggest concern about starting training this early is not physical, it's whether you will destroy a child's desire to play that sport after you have taken away his or her childhood. Children as young as five years old are shipped off to Florida where they have private tutors and play tennis for several hours a day. Young football players are encouraged to take supplements. Gymnasts work out for six hours a day and are told to restrict calories. Try not to be one of those parents or coaches who place athletic training above the child's best interests.