Please, DON'T TOUCH

Its simple tell the kids parents that the stereo is off limits

The key is to avoid to tell them DON'T TOUCH. No matter what way you mention it they would feel uneasy to stay at your place.
Disassemble the whole system and put them in the corner with drap cover it.

Put the kid up for SALE on Audiogon !!!!

George has it right! Fageddabout diplomacy or psychology or instruction-giving. UNLESS YOUR VISITORS ARE ALSO AUDIOPHILES, lock off the living room, or lock up the equipment in a closet.

I think the demo is a good idea. I would consider movers blankets over the speakers. Better yet keep the kid out of the room unattended. Remember some 6 year olds may be too young understand don't touch. I was fortunate that my kids never damaged my systems. My cleaning lady did rip the stylus off of two turntable! while dusting. I never wanted to freak the kids out over the equipment I always want them to feel more important then the stereo. I do joke with them all the time that the stereo is my third child # 1 of 3. My son blew a fuse when he was very little turning the volume to Max by accident. He cried but I laughed and told him not to worry just be careful. He now composes classical music in high school that is amazing and knows classic rock like an enclyopedia. And loves his awesome vintage Music systems a/d/s, Revox , Tandberg, Linn LP12 etc.! Lastly kids need activities like going to the park or being envolved in stuff or they find things to do like drawing on walls or "playing" with your stereo. Keep them busy!