Karate, Tae Kwon Do, Judo - what's the best martial art for kids?

In a nutshell, it really doesn't matter. The basic philosophy behind each are fairly similar - the big differences lie in the physical aspects. Karate, for example, uses lots of powerful punches while judo more closely resembles wrestling. Right now, tae kwon do schools are the most popular and probably the best choice for the Power Rangers set - the style is dynamic and easy to learn, but it's not overly demanding on young bodies.