Fuji - I'm inclined to disagree - if someone is PIC, they're PIC. If I'm not happy they can do that job, I'm not there (nor are any kids or family I might have). Equally I'm not having anyone on board who thinks they have the right to intervene with the flight (advice I'm happy to take, at my discretion).
Fighting over the controls is not a healthy picture, nor likely to have a happy outcome.
Mark - you are correct, even more certainly when it comes to kids, family etc where they are not pilots.
As to someone intervening it is always a matter of last resort, and obviously there is more than an element of judgement. However I am not going to have someone fly me into the side of a mountain, power cables or spins. I have been in that situation on the first two. Unfortunately it is not always possible to be certain in advance that a pilot will be able to cope with every situation. You might not be happy with the intervention BUT if you think the intervention cant do any harm even if you disagree at that moment in time it is as well to find out rather than refuse to transfer control.
A good example - not visual, go around. Handling pilot does nothing. I said again - go around. Still does nothing. At that point I have control, full throttle and around we go. If I hadnt we would both be dead, with hills immediately beyond the go around point whatever the pilot thought, I am not hanging around to enjoy the debate and to find out I was right. I know that sort of thing should not happen and when you are not visual at the DH there should be no question from the handling pilot as to what he should do - unfortunately life is not quite like that. Neither of us had anything to lose by the handling pilot not accepting the go around call. Neither of us had anything to lose by transferring control. Having gone around we both now have the luxury of debating what went wrong. The alternative was to leave that luxury to the AIB. If I had got the wrong DH so what - nothing lost than pride, if the pilot has got the wrong DH nothing lost but both our lives.
If you ever find yourself in that situation please for me intervene and forget about all those stories of very annoying instructors and friends who want to intervene at every possible opportunity for no good reason. Back on the ground if you still disagree with me, by all means never fly with me again.