I think you have to accept that children under say 5 will want to touch, and to some extent I encourage it. I run locos and rolling stock accordingly, usually just an LGB MOW with the overhead removed, which is more or less indestructible and importantly very easy to rerail, and a wagon. Sometimes I run an 0-4-0 steam shunter with a sound card and with the couplings removed (ref OP). I have a pile of Playmobil people and animals that I let them load onto the train. I ran a couple of sessions for my wife's toddlers group recently, 1-4 year olds, no serious damage except they seem to be fascinated by the little stones making the track ballast (Alpine grit) and feel they need to load it stone by stone into a wagon, so some re-ballasting required. They like the locos disappearing into the shed and reappearing again as if by magic. This is all on my small outdoor track. I use an old iPhone as my outdoor controller and if the children are old enough to understand smart phones (about 8 years old these days) then I let them take turns driving a loco. For demos of the serious stuff (adults only) I use the indoor track where it is definitely hands off unless invited.