Girl Guides are like Boy Scouts, it's a lot like herding cats. A helpful hint from an old scouter, try to burn off some of their nervous energy before you get into showing them a few moves. Start them off with a brief talk and as they start to show the signs of boredom just transition right into some basic warm up moves before you show them 'the real stuff', 5 minutes of warming up (for safety so they don't pull any muscles) should get them pink in the face and refocused for the demonstration.

But it’s really important. If it’s better, request that they divide into two smaller classes.