i start my clinics out with a simple 2ppl in, two forhand, two backhand then rotate drill. the key is to get then hitting balls. they pay to play tennis not to hear u talk or strech.
do a few drills, play a few games. too much of one thing or another will bore them.
if ur nervous u have to hide it, those kids will eat u alive
even if u pick a bad drill/game, stick with it til the basket is empty. changing the drill quickly after u called it makes u look bad. if u get really desperate do an overhead drill, kids luv that.