No it won't. There is no evidence to suggest that strength training affects children negatively.
Many respected bodies not only support children lifting weights, but recommend it. A few of these bodies are:
Sports Medicine Australia
American College of Sports Medicine
American Council for Paediatrics
Society for Paediatrics and Orthopaedics
American Academy of Paediatrics
National Strength and Conditioning Association
National Athletics Association
United States Olympic Committee
The Presidents Council for Physical Fitness and Sports
Current research now conclusively shows that strength training in children has no adverse effects on bone growth, growth plate integrity, muscle development, bloody pressure, heart rate, haemoglobin content, flexibility or motor performence. Research has also shown significant improvements in strenths equal to that of an adult.
The American College of Sports Medicine actually suggests that if a young athlete is not peforming strength training, their coach or parents are actually neglecting the health of the child due to the increased potential of injury from their sport.
So get out there, and start lifting. Just remember for children:
Warm up and cool down properly
Train only 1 or 2 times per week
Lift at a controlled speed while maintaining proper form
Use high reps - above 10
Do not take sets to failure until child has been training for a long period of time and established high levels of strength.
