What a load of boll*cks, your children, spouse or friend are not acting as "flight crew" or "pilots" under Art 50 just because they might briefly have a go at flying your own aircraft or one you have rented, they are neither under any official instruction to gain a licence and while they are having a go you are the official pilot in command of the private flight.
Practically this might indeed be "a load of boll*cks" but, legally, it most definitely is not.
A pilot is defined (not by EASA nor the ANO, because they give no definition) as either:
1) Someone who is qualified to fly aircraft
or
2) Someone who controls an aircraft in flight
A person can certainly do 2) irrespective of if they are 1)
Think of a young, unlicensed, joyrider. They were certainly the 'driver' of the car and, if they are caught, they will receive points on their non-existent Licence.
Now think of an unqualified person stealing an aircraft. They are certainly 2) 'acting as a pilot' and, if solo, they are also 1) acting as PIC.
The two are not the same.
So, an unqualified person handling the controls in flight, with the permission (and under the supervision) of the PIC is certainly 2) 'acting as a pilot' - especially if they dive the plane and go "daga, daga, daga".