Wing Chun Kung Fu is Chinese in origin, and was originally practiced, I beleive, by women monks to defend against attack. Wing Chun has a nice fluidity to it, focuses on balance, and is based at least in part on the principles of yin & yang. As applied, when attacked aggressively one should apply a passive force to simply redirect the opponent's energy in order to move him/her off balance; when the opponent becomes unbalanced (passive), it is time to move in with force. Perhaps a nice life lesson for your 7-year old.
I have practiced Aikido as well. It was interesting and athletic, though it did not speak to me personally like Wing Chun did.
However, I would suggest the better question to ask is "who is the best martial art teacher?" for as with so many things, it's the professor -- not the subject -- that makes a great class.
I would also encourage you to consider learning the martial art alongside your son. I learned with my brother, and 20 years on it's still a language of physical movement that we share.
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