Susan has hit it bang on :
...check around...talk to them...watch one of the classes...Young kids have the attention span of a gnat...so great patience and short classes would be required...
Classes adapted to young kids is a must. Other than that, everyone will preach for his favorite martial art. I do Aikido and it takes years to master, I like it's non aggressive philosophy. I wish I had started 30 years ago ... Starting young is a tremendous advantage ...