I've been teaching art to kids for almost 30 years. The only advice I can give you is say little and do lots. A lot of talking at the beginning of a new venture is a hurdle kids won't appreciate. Let them get out there and make mistakes. It is only when a kid wonders why something happened that he/she is really receptive to hearing about it. The "need to know" principle means that when someone decides its necessary to know something, because they want to accomplish some task that they have defined as important, you will only have to explain it to them once. (OK, twice... they're still kids.)
More important than anything else - have fun! It's why you do this thing, isn't it? Pass on that passion and everything else will take care of itself.