No, not really. Anything can be bad if you screw up hard enough, but there is nothing inherently wrong with weight lifting for kids. Many official boards of athletics and exercise physiology approve of it as an alternative to youth activity, and weight lifting does increase strength and endurance in youth while reducing body fat. If your child wants to lift weights, it should be fine; just make sure they know how to do it right.
Official stances of approval include the Canadian Society for Exercise Physiology which tout that "(Resistance training) for children and adolescents can be relatively safe and improve overall health", the National Strength and Conditioning Association also is in approval for properly conducted weightlifting as a feasible alternative for youth activity.
Weight training, for children, doesn't appear to cause significant muscle growth like it does in adults. That being said, it can increase muscular strength and endurance. A 'higher rep' range of 6-15 repetitions seems to be most effective.
That being said, one meta-analysis did suggest that it is hard to get quality data due to methodology in studying this topic.
Resistance training can decrease fat mass in children and be a potential way to combat childhood obesity and can have effects in as little as 8 weeks. When constructed properly, they can be very safe as well.
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