Why not just go to the experts?
http://www.webmd.com/parenting/features/is-weight-training-safe-for-kidsHow old should a child be before they start strength training?
At least 7 or 8, McCambridge says. It takes that long for the child's balance and posture control to mature, according to the AAP.
The child's age also affects how much weight they should use. ''The younger they are, we recommend light weights, proper form, higher repetitions," McCambridge says.
But children 7 to 8 probably don't ''need'' to strength train to have a well-rounded physical activity regimen, McCambridge says.
Weight training programs must be appropriate for the age and development of the child. Supervision is crucial, especially with younger children.
Free weights may be better than machines, which are typically designed for longer limbs, says Debi Pillarella, a certified personal trainer and fitness program manager for the Community Hospital Fitness Pointe in Munster, Ind., and American Council on Exercise spokeswoman. That may change, she says, as "some companies are making youth-sized equipment."