Most of the time the only kids that have any access at all to my layout are the grandchildren, and they’re not a problem. I let them put their dolls, dinosaurs, or whatever on the gondolas for a “ride on the train”. I also allow them to run the trains, after receiving proper instruction. My trains can’t go very fast anyway.
Though of course, sometimes accidents happen. I expect that so I’m not too freaked out if it does. My biggest concern is falling, since there are steep stairs, uneven paths, and nothing to land on but rocks and concrete.
When we have guests with children, the only problem I’ve had so far is that they tend to get excited following the train back and forth, and before you know it they’re running.
Sometimes if the guest’s child is well-behaved and interested, I’ll let them have a go at running the train for a bit. We had one little boy who was a big Thomas fan and really hooked on trains, and at the age of four was very responsible about running them!