I started in the martial arts when I was five. Not really BJJ back then, but more of a karate base.
It was the best thing I ever did in my life. The key for kids is making it fun, so try to keep it interesting and just go over the basics. Make up some fun drills and present them as games instead.
All in all, kids and adults aren't that different. If you just try to think about what you would have wanted to do when you were a kid, you will be fine. Kids like to have fun, but so do adults. A good adult BJJ class will have fun drills and techniques that keep things interesting. Try to find similar fun stuff and kids will respond to it.
Also, I think it's okay to teach submissions to young kids. I would not let them crank it hard on each other at first, but then again I would not let adults do that either. Just make sure they understand how dangerous they are and that they are not to be used outside of training.
Once kids reach school age, they seem to realize that they're actually capable of hurting others. I know I was aware of that at that age. Even if you don't teach a kid submissions, he still knows that he can hurt other kids if he wants to by smashing them heavy things, stabbing them with sharp things, etc.
The fact is, unless a kid is crazy, he doesn't want to hurt others, so as long as he understands that submissions hurt, I think it's fine to teach them.