Never heard that one in 30 years of flying...
Agree with AB here. I'm a pilot and volunteer with the Dutch charity "Stichting Hoogvliegers". We openly advertise that the chronically ill and handicapped children we fly with, are going to handle the controls once we're established in the cruise. We have openly discussed this and other matters with the Dutch authorities and they have no objection whatsoever.
Of course you need to apply common sense. For starters, we require that the children we fly with are able to understand spoken commands (in Dutch) and are able to execute these. We don't fly with children up front who might be exhibiting unexpected/uncontrolled behavior or who might have suicidal tendencies. We provide each pilot with a "bag of tricks" on how to deal with their passengers, how to subtly keep control of the aircraft while giving the impression that the passenger is flying, and how to get an unwilling passenger to relinquish control without resorting to violence. And we do stress that in the ultimate case they are allowed to resort to violence.
It is absolutely illegal to operate an aircraft without the necessary qualifications (licenses and ratings)
True. But "operating an aircraft" is a far broader subject than "manipulating the controls".
If letting a passenger having a go at the controls would be illegal, then we would be openly responsible for 5000+ breaches of the law. The evidence is out there on our website in the form of 5000+ trip reports so the government would have no problem prosecuting us. That this hasn't happened must be telling something.