Ditto on the USA Martial Arts dojo at Spring Creek and Coit. We live closer to the one in Allen, but we were so impressed with the instructors at this site that we allowed my son to enroll. We went to a birthday party there for a preschool classmate and our son (4.5) reacted so well to the lesson and showed an immediate interest. As Amy indicated, it's much, much more than just about martial arts at this location. The focus this month was good manners and they talked about different kinds, i.e., playground manners versus eating out manners. This is in a short 5 minute talk at the end of class. The rest of the time is spent learning techniques, such as punches, kicks, getting out of holds, etc.
Good luck with whatever location you choose, but pay attention to how the adults interact with the children and one another. Your child will look up to their instructor (just like they do to you), so it's important they are exhibiting behavior and characteristics you want your child to model.