I feel it mostly depends on your Sensei. Don't get one of these yelling all the time, and very abusive, strict types. You want one that is good for the kids, and makes it fun , but as they grow, he toughens up with them.
Go to a few classes, and watch with your kid. See what he thinks of the Sensei, and how things run.
Don't only go to one class to make your decision. In our Dojo, their are different things reviewed on different nights. You may go on a night where it looks great, or a night where it looks bad.
I have seen where kids will quit because they feel their Sensei is mean. .
You want them to enjoy the learning experience. Martial Arts can be very fun, and very emotional. It is great excercise, a great confidence builder, and really teaches discipline.
Kempo is very good, because it focuses on the empty hands, and feet. Punches, and kicks. There are a few take downs/sweeps, but not like Judo.
I would suggest going to a few different dojos, and deciding what style, and Sensei is best for your child.
Also, I am not sure about age limits, I think it is more of a decision you, and the Sensei make of is he/she ready.
My almost 5 year old wanted to try, but she wasn't ready. My wife wants her in dance anyways.:(
Yet there are some that aren't five yet, that are pretty good.
It is an individual thing.