Yes, In fact it is very good for them when properly supervised.
Please refer to the following current information on the subject of
kids and weightlifting.
Pediatrics, Volume 121, Number 4,
April 2008 policy statement of the American Academy of Pediatrics
Applied Physiology, Nutrition and Metabolism, 33:547-561,
2008 policy statement of Canadian Society for Exercise Physiology
Journal of Strength and Conditioning Research, August; 23(5): 560-579,
2009 policy statement of the National Strength and Conditioning Association