insidious_machinae

I would avoid having them learn any of the more aggressive martial arts. This includes karate.

Judo means "the gentle way" for a reason. You don't kick, punch, jab, slap, or anything of the such. Basically all a 5 year old kid would learn is a little discipline, and how to do some fun rolls on the ground, and maybe learn to take a few falls. I used to work with the little kids at my dojo, and it was always a blast, both for them, and me.