Did a Google search, found this:
"Running and jumping alone puts a force more than 5 times a person's weight on one legs. If a man is 200 lbs, he'd have to be squatting more than 1,000 lbs in order to be putting more stress on his legs than regular running or jumping would do. If running or jumping doesn't stunt your growth, neither will doing squats."
Wikipedia article on weight training (http://en.wikipedia.org/wiki/Weight_training) addresses this also:
"Orthopedic specialists used to recommend that children avoid weight training because the growth plates on their bones might be at risk, but recent studies have shown that this concern is unfounded. The very rare reports of growth plate fractures in children who trained with weights occurred as a result of inadequate supervision, improper form or excess weight. "Growth plate injuries have not occurred in any youth strength training study that followed established training guidelines." The National Strength and Conditioning Association also confirms that "a properly designed and supervised resistance training program is safe for children."
Sounds like form is key, as always.