A few years ago our team took our flight into a local airport for a public 'airport days' fly-in. We parked our aircraft next to some other RV's and took turns guarding the hardware.
I discovered one kid hanging off the prop of our flight lead's RV. I gently educated the mother on how dangerous it was to be paying around airplanes... the engine could start, or a spark could ignite fuel vapours, or that smoker over there sitting under the wing.... oh shzt!
A grown man had set up his lawn chair next to an RV, within 24 inches of a dripping fuel vent. The grass was dry and crispy and he was smoking a cigarette! At that point, I simply asked him to never smoke on the airside of an airport, and the example I used was the dripping avgas on the grass next to his chair. To his credit, he extinguished his cigarette, but I kept my eye on him for quite a while.
At this point, I have a policy of never going to a fly-in event where the general public has access to the aircraft unless I am prepared to guard my own aircraft the whole time or there are ground crew to do the same.
On the other hand, I am participating in a "Young Eagles" style event next month where kids and parents will be introduced to the magic of flight... complete with ground school training about aircraft and flying and with adult supervision.
It's perfectly acceptable if you see a kid with parents at one of these events to invite them to have a closer look. Begin with the safety briefing (prop, dripping fuel etc.), follow up with how easy aircraft are to damage and give them the professional treatment and a photo op. If enough of us do this, the kids and parents will learn how to behave around aircraft, and maybe tell their friends, too.
It would also be nice if the organizers of these events have group 'ground school' for the families... 5 minute safety briefing and they get a pin!
...The worst incident I had of disrespect was from a fellow RV pilot at a local fly-in. All of the aircraft were lined up tail-to-tail in rows and this fellow decided to do his runup while still in line. My aircraft was directly behind his, and several of my friends were frantically signalling to this fellow to cut his engine, while others were trying to stabilize my RV. We now park the aircraft differently at this event.