This thread reminds me of a certain U. S. submarine off the coast of Hawaii which was flashing by having a dozen or so civilian guests aboard during practice dives and surfacing. Some said that the guests distracted and crowded the crew and thereby may have contributed to the accident and the fatalities that resulted. Is such a distraction, even if very unlikely aboard an aircraft, worth taking the risk just to please an adult or some children who otherwise would not know the thrill they had missed? I don't know the rules re visitors to the Flight Deck, but I side with the first poster and would feel more comfortable if I could read official rules that state that an adult can take two children into an active Flight Deck with him.