There are several reasons to become involved in strength training, including overall strength improvement and even helping to improve recovery after an injury.  It is important to note that weight lifting and strength training are completely different.  Weight lifting is dangerous for children, while strength training is perfectly safe if properly supervised. 

Strength training should never be your child's only sport activity, but rather in addition to their other activities.  There are several benefits to strength training for children including increased muscular strength, balance, strength around joints, and endurance.  Additional benefits are improved sports performance, posture, and helps prevent injuries.  Strength training is also helpful to aid in speeding the healing time after an injury. 

Children as young as 7 can benefit from strength training, remember that strength training should be in addition to other activities such as baseball or gymnastics.  You should ensure your child's instructor is trained and experienced working with children your child's age.  If the trainer has only worked with adults then your child should not work with them, there are major differences between children and adults.  You should look for an instructor who is certified by the National Strength and Conditioning Association (NCSA); the instructor should have either a CSCS (Certified Strength and Conditioning Specialist) or a CPT (Certified Personal Trainer). 

A CPT is an instructor who works one on one with clients, while a CSCS is someone who designs and implements safe and effective strength and conditioning program.  A good program for your child should practice no more than three times per week, with at least one day off between sessions to rest.  You want a program that features higher numbers of repetitions with low weights, rather than few repetitions with high weight.  Muscle mass is not the goal of strength training.  Children can easily do resistance exercises such as push-ups, sit-ups, squats, arm curls, calf raises, and pull-ups. 

Your child should not start using weights until after puberty.  Once puberty is started it is possible to start adding muscle mass.  However, free weights should never be used until your child starts ninth grade.  Resistance weights should be added much later, once your child has mastered proper form and technique it should be safe to add additional weight to routines.

With appropriate training and supervision strength training is very safe, and has very few injuries each year.  A child should never be unsupervised with weights to avoid injury.  The costs associated with strength training are fairly low, costing about $15 per group class with private lessons costing between $20-$100 per hour.