As far as airside safety is concerned, we run 15 charity events annually with each welcoming between 25 and 100 sick/handicapped children with their parents.
We create a physical barrier between airside and landside (using portable fences if possible, with red/white tape if nothing else available) with a "gate" in it, guarded by volunteers.
We only allow our guests airside if they are either accompanied by "their" pilot or by an airside marshall (typically a pilot who is not flying that day). And there's a selection of other personell, mainly photographers, who are allowed airside independently.
Everybody who is allowed airside independently (pilots, marshallers, photographers) are required to wear a high-viz vest. This is not because we believe the vest in itself prevents accidents (our events are only held during good Day VFR conditions, where conspicuity is not really an issue), but it allows us to glance at the airside operation and know everything is "in ordnung": Every individual not wearing a vest has to be supervised by someone with a vest. Easy.
Obviously this is augmented by procedures that allow the pilot to communicate whether a prop is "hot" or "cold" and a few other, minor things.
But they physical barrier between airside and landside, the insistence that everybody airside has to know what they're doing or has to be accompanied by someone who does, and the hi-viz vest policy are the most important measures. We are close to having held 100 events by now, all without airside accidents.