Is weight lifting bad for kids?

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.

Safety and Injury

Official stances of approval include the Canadian Society for Exercise Physiology[1] which tout that "(Resistance training) for children and adolescents can be relatively safe and improve overall health", the National Strength and Conditioning Association[2] also is in approval for properly conducted weightlifting as a feasible alternative for youth activity.

Efficacy in Muscle growth and strength

Weight training, for children, doesn't appear to cause significant muscle growth like it does in adults.[3] That being said, it can increase muscular strength and endurance.[4][5][6] 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.[7]

Potential benefits

Resistance training can decrease fat mass in children and be a potential way to combat childhood obesity[8][9] and can have effects in as little as 8 weeks.[10] When constructed properly, they can be very safe as well.[11]

References

1.^Behm DG, Faigenbaum AD, Falk B, Klentrou PCanadian Society for Exercise Physiology position paper: resistance training in children and adolescentsAppl Physiol Nutr Metab.(2008 Jun)
2.^Faigenbaum AD, Kraemer WJ, Blimkie CJ, Jeffreys I, Micheli LJ, Nitka M, Rowland TWYouth resistance training: updated position statement paper from the national strength and conditioning associationJ Strength Cond Res.(2009 Aug)
3.^Behringer M, Vom Heede A, Yue Z, Mester JEffects of resistance training in children and adolescents: a meta-analysisPediatrics.(2010 Nov)
5.^Payne VG, Morrow JR Jr, Johnson L, Dalton SNResistance training in children and youth: a meta-analysisRes Q Exerc Sport.(1997 Mar)
6.^Faigenbaum AD, Loud RL, O'Connell J, Glover S, O'Connell J, Westcott WLEffects of different resistance training protocols on upper-body strength and endurance development in childrenJ Strength Cond Res.(2001 Nov)
9.^McGuigan MR, Tatasciore M, Newton RU, Pettigrew SEight weeks of resistance training can significantly alter body composition in children who are overweight or obeseJ Strength Cond Res.(2009 Jan)
10.^Sgro M, McGuigan MR, Pettigrew S, Newton RUThe effect of duration of resistance training interventions in children who are overweight or obeseJ Strength Cond Res.(2009 Jul)
11.^Sothern MS, Loftin JM, Udall JN, Suskind RM, Ewing TL, Tang SC, Blecker USafety, feasibility, and efficacy of a resistance training program in preadolescent obese childrenAm J Med Sci.(2000 Jun)