Last year at one of our once-a-month open to the public running days, a young autistic fella reached into the cab of my 1/3rd scale diesel loco through the open windows and trashed the entire scale fitted control panel in less time than it took one of us to reach him. The cost to replace the broken bits was not a lot, to tell the truth, since most of the levers were actually small operating knobs from farming equipment, but the shock to the system was a real wake-up call to us all. Thankfully he stayed well away from the 1/3rd scale live-steamers, else it could have got very painful very quickly.
Earlier that year, two visitors - both of whom declared themsleves expert operators - succeeded in almost destroying a Gauge 1 live-steam German Pacific locomotive by letting it boil dry without rurning off the gas....they then walked away and left it cooking.
So now, hard-nosed it might seem, but unless their level of experience in train operation is known to me, the only thing I'll ever let visitors drive on my track at home is an LGB track cleaner.
None of the steamups I've been to in the last dozen years allow anyone other than the owner of a piece of equipment to run it, unless said owner grants permission to a friend, relative or hanger-on.
Children are naturally curious and want to touch the trains. They need nearly constant reminding not to touch live steamers. The good news is that most children will pay attention and follow instructions; their parents can often be another story.
Some of the more hair-raising examples of guests run amok that have been cited in previous posts give ample reason to consider that if you're hosting a get-together of any kind, be certain that your liability insurance is up to date. (No, I do not work for a company which sells liability insurance). Unfortunately, it's all too common in the US for visitors, guests or even people who casually wander onto your property to sue, sometimes many weeks or months after the alleged injury.