I'll copy/paste exactly what I posted in a thread about this from last year...
About 10 minutes of searching the web, and I found the following:
"This study examined the safety of one type of strength training for prepubescent males. Eighteen males (average age, 8.3 +/- 1.2 years) participated in a 45 min/session, three session/week, 14 week supervised strength training program with an attendance rate of 91.5%. Concentric work was done almost exclusively.
Results showed that in the short term, supervised concentric strength training results in a low injury rate and does not adversely affect bone, muscle, or epiphyses; nor does it adversely affect growth, development, flexibility, or motor performance. As the safety question is multifaceted, this should not lead to the conclusion that strength training for prepubescents is uniformly safe. Further research is needed."
"Two socioeconomically equivalent schools were randomly allocated to be an "exercise" or "control" school. Twenty boys (mean age 10.4 years, ranging 8.4-11.8) from the "exercise" school participated in an 8-month exercise program of 30 minutes of weight-bearing activity three times weekly for 32 weeks (basketball, weight training, aerobics, soccer, volleyball, gymnastics, folk and line dancing)...
Most anthropometric measurements increased during the 8 months (Table 1). The increases in biacromial and femoral intercondylar widths in the exercise group were greater than in controls. BMC and areal BMD increased in both groups at all sites except the arms and skull. The increases in the exercise group were twice those in the controls at most sites, reaching statistical significance at the lumbar spine, legs, and total body areal BMD"
(Translation into non-science speak: Weight-bearing activity, which included weight training, significantly increased bone mass density and and bone mineral content in the exercising kids.)
2003 journal from the peer-reviewed The Physician and Sportsmedicine (PDF file):
"One theoretical concern is that the growing bones of children may be less resilient to physical stresses than the bones of adults. Although a few case study reports have noted growth plate fractures in children who lifted weights, most of these injuries occurred as a result of improper training, excessive loading, and lack of qualified adult supervision.
A literature review reported no cases of any overt clinical injuries, including epiphyseal fractures, among those in appropriately supervised strength training programs. The risk of an epiphyseal plate fracture in prepubescents is actually less than in adolescents, because the epiphyseal plates are stronger and more resistant to shearing forces.
Recent literature indicates that strength training will not have an adverse effect on growth. A few studies have shown positive growth effects as long as proper nutrition and age-specific physical activity guidelines were met. However, resistance training will not affect an individuals' genotypic maximum. Parents can be assured that strength training (in moderation) will not have an adverse effect on growth. Training may actually be an effective stimulus for growth and bone mineralization in children, especially for those at risk for osteopenia or osteoporosis."