Even though I teach kendo, I recommend judo for kids that young. They have lots of fun wrassling, they don't learn kicking or punching that might get them in trouble in the schoolyard if used, there's lots of variety in the techniques so they don't get bored, it's great physical conditioning and they learn how to fall properly. Judo is widely available, cheap like borscht and mostly taught by dedicated volunteers who are there because they want to be. There's nothing they learn in judo that will screw them up for another martial art later on if they decide to take a different direction.
Neil Gendzwill
Saskatoon Kendo Club