Having these layers will help make the composition more interesting. If you have a very large group you can climb a ladder, or onto a roof and shoot down on the group. That makes it much easier to build layers. They won't understand and may complain. Keep the group as close together a you can, but make sure that you can see every face and that no one is completely or partially obscured by anyone else.
Kids are useful for the trickier spots and to fill in holes in the group.
Remind your subjects that if they can't see you, you can't see them. They will no doubt gripe about how long it's taking. Take a couple of photos, stop and look again at the group, checking for squinters, hiders and those for whom the two-minute break from conversation has proved too much to maintain. Demand their attention and then take a couple more. This is a tacit acknowledgment that you just may know what you're doing.