Not necessarily. As long as the child is getting enough sleep and a balanced diet with enough calories to compensate for the additional exercise, growth should be normal. However, if nutritional intake isn't enough to compensate for the calories being burned by exercise, or if an eating disorder develops, growth and pubertal development can be affected.
No. Intense physical activity in children does not usually affect growth negatively. It may delay the onset of menarche (her first period) in girls. But that can happen in other sports such as running as well. The biggest problem is over training with subsequent injuries. The rise of year round youth sports has lead to an increased number of overuse injuries.