My kids are under 10, so washing the grubby little hands is rule #1, then no eating or drinking. I have an enlarging station set up for them with their own enlarger, light box, timer, grain focuser etc. Their attention span is like 15 seconds, so I don't have them down there long. I set some time aside and bring them down to each do a print. I use the Kodak projection print scale, so they do the exposure on RC paper and I quickly run it through the chemicals and dry it for them and they get to pick the slice they like. They set the timer and focus and make the print. They are read to go play something else by the time the prints are in the fixer so I let them go and give them the prints the next day. So far it has been a great experience.
I have the enlarger on a standard height counter, but it is too high for them, even for 4x5 enlargements.