If you can't find a qualified instructor (try
www.kidscansew.com and
http://www.palmerpletsch.com/teacherlist-CSI.htm ) start them sewing on paper - draw some straight lines and have them line up the edge of the presser foot with the lines and sew (no thread, of course!). As an instructor (
www.sewyoucan.com), I've found that kids look at the needle and go way too fast...and create a huge mess. And keep the projects simple - their attention span is very short and sewing involves a lot of pointy, potentially dangerous objects. We typically start with lunchbox napkins, pillow cases, and tote bags.
But I would really recommend finding someone who is a professional. Knowing how to sew and teaching sewing are two different skill sets.
Good luck!!