That is a LOT of responsibility, even if you are getting paid.
We hired a wedding/reception coordinator, whose responsibilities pretty much included making sure that the wedding party was in the right place at the right time, making sure the chairs were set up, the aisle runner was set, etc. At the reception, the caterer handled all the food issues, the coordinator just made sure that we stuck to the schedule for things like cake cutting, toasts, etc. No corralling unruly children involved. We paid $750 for her service, and she was one of the less expensive ones out there. We do NOT live in a high cost of living area, either.
I guess, if this is something that sounds like it would be fun/manageable for you to do, go ahead. But, if you don't ask for payment, you're essentially giving them a ~$1000 wedding present. I would probably respectfully decline.