My kids have been taking Taikwando since 3rd or 4th grade (several years now) and I agree 100% with what Susan said. Its not so much the type of marshal art, but the sensibility of the instructor when dealing with young children. As a parent you'll know what is right for your child.

We lucked upon a great insructor that taught classes at several day care centers adn then decided to teach at our (his too) elementary school. The progress is slow, but steady, which is all I expect.

No 3-6 year old is going to be able to take on an intent adult, especially one with lethal weapons, but it is a great way to get them excercise, teach them self and situational awareness, self confidence, and eventually protect themselves.