My two oldest children started with the Nackord Kenpo Karate System at the same time, when they were six and four. A few years later my youngest started shortly before his fourth birthday. Initially, karate was just a fun activity, but soon they realized how much they enjoyed it and how much they were learning. It is now over ten years later and they still come regularly every week--and they continue to learn. More importantly, it has become a valuable part of their lives in many ways. It gave them a strong sense of self-confidence, it reinforced their respect for themselves and for others; it has helped mature their social skills (around adults as well as children), and it has been instrumental in them achieving and maintaining a high level of physical fitness.
From a practical level, the martial training has even helped them in conflict situations. First and foremost, they are comfortable enough to not feel that they have to engage in a fight. But even when that has not been the case, the training has paid off. One particular example occurred when my oldest was about 10 years old. He was playing with his brothers and several friends when a bully started harassing him. I happened to be in the area at the time so I was able to watch the situation unfold. The bully, who was significantly bigger than my son, pushed him several times. My son told him stop each time, but the bully ignored him. Finally, as the bully attempted to push him one more time, my son effortlessly grabbed his hand, put him in an arm-bar, brought him to his knees, and told him, "I told you to stop. OK?" The bully immediately apologized, saying that he was only kidding, and the incident was over. So, because of his training, my son was able to handle a situation that would not end otherwise--without resorting to hitting or hurting.
I recommend the NKKS to children of any age and sex (as well as to adults). It is great training, it builds self-confidence, respect, and physical well-being, and it gives valuable training for potentially troublesome situations.