Completely agree with Bob.
Most of the responses here contemplate establishing a very negative interaction with the public. That certainly will not win aviation any friends. I hate to say it, but many GA pilots come across as gruff, arrogant and unfriendly. I really feel bad for the kids and young people on the other side of the fence who are just dying for some positive interaction, and who are usually ignored or worse yelled at.
Let's not forget who pays for many of our airports and who can vote to kill them. That kid who wants to climb on your pride and joy could be the next Blue Angel, military aviator, Rob Holland or fly-in buddy etc. How about roping your plane off with a sign along the lines of "ask me if you want to get closer -- happy to do a tour. Text me if I'm not around and we'll get together. Kids without muddy shoes get to sit in the cockpit." Last kid I spent a lot of time with is now a FO for Cape Air and occasionally still my flying companion.
This will fall on deaf ears for a fair number of you, but hopefully the rest will take it to heart and become aviation ambassadors.