However, governing bodies such as the National Strength and Conditioning Association, the American Orthopedic Society for Sports Medicine, and the American Academy of Pediatrics all suggest that children can benefit from a properly prescribed and supervised resistance training program.

In fact, there is research to support each of the benefits from following a properly designed resistance training program.

Increased muscular strength and muscular endurance
Decreased injuries in sports and general recreational activities
Improved performance capacity in sports and recreational activities
Support bone development

What is most important, is to ensure a safety first approach meaning children need to learn about proper form and lifting techniques and should not be sent blindly into a weight room. They need to have competent supervision and be given a properly designed fitness program.

I always highly encourage parents to talk with the kids about learning body weight exercises first, you should not even use weight unless you have mastered the body weight version first - and this goes for adults and children.

For example, there is no need to get under a barbell for a chest press if you cannot comfortably do 10 or 12 pushups, using perfect form.

The best advice for anyone, children or adults, who are starting an exercise program and looking for general health improvement, is to work on full body movements, through the greatest ranges of motion possible. Learn to do a pushup properly, a chin-up and pull up, squat, deadlift, dips, and lunges.

These all work large muscle groups and help build a strong foundation. Again, with children the emphasis needs to be on perfecting form - this means taking the ego out of the picture, not worrying about how much weight they or others are lifting, practicing proper movements, and of course focusing most of the time on general physical every single day, rather than sedentary down time.
 
In addition, don’t push a child into resistance training, but encourage them and work with them . Before starting any resistance training program at all, here are a few basic guidelines for children and adolescents of various age groups; they are loosely adapted from the book Strength Training for Young Athletes, written by world renowned strength coaches, William Kraemer and Stephen Fleck.

Ages 5 - 7: Introduce child to basic exercises with little or no weight. Teach technique, progress from body weight calisthenics, low volume.

Ages 8 - 10: Continue to practice technique, use simple exercises, and monitor tolerance to exercise stress.

Ages 11 - 13: Teach all basic exercise techniques and introduce more advanced exercises, using little or no resistance.

Ages 14 - 15: progress to more advanced exercise programs, always emphasizing exercise techniques.

16+: Entry level into adult programs assuming a solid foundation has been gained.