While growth plates are not fully developed and still cartilaginous, they could be susceptible to excessive forces and could cause problems with ligaments or fractures. When a child is ready to participate in organized sports, it is acceptable to begin a program that focuses on tone and flexibility. Any weight resistance used should be low weight/high repetition. The child should be able to lift the weight ten times without strain. Any strength training must be well supervised and I would recommend consulting with a professional trainer before designing any program for a pre-teen. When done correctly and with safety in mind The American Academy of Pediatrics says that strength training may lower the risk of injury in certain sports. For more on this topic go to:

http://cbass.com/Weights_kids.htm

Age two is too young for any actual resistance training, though there are plenty of exercise options that will encourage health and fitness until they are ready to advance.