To keep in shape, is not only an issue for adults. The children of today eat lot of fast food, they sit still and many children are over weight.
Strength training is a great form of training that helps the child keep a healthy weight.
Read more about Children's Training here:
Canadian Society for Exercise Physiology Position
Resistance Training for Children an Adolescents
Strength Training by children and adolemscents
References:
Strength Training for Children and Adolescents
Jeffrey A. Guy, MD and Lyle J. Micheli, MD
J Am Acad Orthop Surg, Vol 9, No 1, January/February 2001, 29-36.
STRENGTH TRAINING FOR CHILDREN AND ADOLESCENTS
Avery D. Faigenbaum EdD*
Clinics in Sports Medicine
Volume 19, Issue 4, 1 October 2000, Pages 593-619
Mayo Clinic
http://www.mayoclinic.com/health/strength-training/HQ01010