Have you checked with your Principal? There may already be guidelines in place regarding volunteers at the school. That would also include PTO/PTA sponsored events held on campus.
I would also contact your insurance carrier and see if they have any guidelines for coverage. Either one of those would be a great jumping off point.

We need to write a set of volunteer guidelines for PTO sponsored on-campus events. We had an unfortunate incident with a volunteer and a non-student child that had been "dropped-off" at our elementary event. Does anyone have any safety guidelines that they would be willing to share?