Although most tennis people know that girls struggle to throw as well as boys, it is something that needs to be trained at a young age. With practice and the correct technical work, girls can throw very well, but it requires the coach/parent to focus on tennis-specific throwing mechanics rather than just throwing the ball forward. The big problem most people have is that they do not focus on throwing with a high trajectory (i.e. long tossing) while also trying to maintain a positive shoulder-over-shoulder position (think a good tennis serve position at contact). If the athlete throws with these mechanics at a young age, she will develop good throwing mechanics that will translate into a more effective tennis serve and should help her also be able to hit an effective kick serve. Take read of the article below from the Washington Post that provides some more statistics.