I have very short tolerance for parents who let their kids run wild around my layout, and some adults are just as bad.
Given that, I have an area just for kids under my layout where they can crawl trough a kid-sized tunnel portal and watch some trains from inside of the mountain. There are some old plastic trucks and tractors in there if they need to touch something.
I ask them if they want to see the inside of the mountain...the answer is almost always yes. I also let them know that only kids can go in there; no parents. They love it!
The parents can safely watch them from just outside the portal.
TJ