Everything so that they have fun, and if they are not same level of ability, so that everyone has portion of fun. Use games with balls, racquets and court rather than teach them strokes. From just getting used to all the stuff progress to games related to basics. Anyway, make them move all class long - it's an activity first of all.
Ah eh and the forehand lag-release fo sure so that they have the correct habits from the day 1!