I'm still way to new at this to give much helpful advice, but I'm finding myself in similar situations and I'm anxious to hear how it turns out for you. I really liked what someone said about going in their confident. If taking money makes you more nervous or erodes your confidence, I'd wait until you deliver the photos or after the shoot to discuss or accept payment. Confidence is key when directing groups of people, and the group will be able to relax with you in control.
I do have one photo tip for you. Actually it's from this forum. Earlier I was reading the Quintuplets Car Wash post and found a great gem called Group Shots. I haven't tried it yet, but it’s a free tool that will create a composite from multiple shots. Setup on the tripod, shoot several continuous shots each time your ready, then later if one of the kids looks away or someone blinks, you can use the composite to solve the problem. Like I said, I haven't tried it but I can't wait to see the results. You can read the post at
Quintuplets Car Wash.
Good luck on the shoot! Can’t wait to hear and see how it turned out.