Once having conceded to the illegal requirement of the child, having given in to feelings of pity, fault or just because it is so much simpler, you give to the kid the first feeling of the real power over the person, in particular over you. Any power corrupts. Also you put the first pebble of a wall which has over time good chances to grow between you. The simple good human contact can be only horizontal, between equal people who respect each other.
What does it threaten the child with?
He studies model of the relations, absolutely new to himself based only on blackmail. Such model can be pleasant how are pleasant, for example drugs because they yield instant result, and together with it a possibility of easy self-affirmation. Over time skills not of game, but open communication with people are forced out by manipulation habits. The abyss between such children and the world gradually grows. In such game there cannot be winners.
Actions of parents in communication with the manipulator
The most important is not to start process. The earlier actions, the better will be taken. Unfortunately, much parents cannot be firm through sense of guilt before the child. It is possible to get rid of such feeling if to make the plan of the own rights and duties, and accurately it it is directed.
Equal communication with the child
Try to forget about role communication. Get rid of image of the tyrant, the tutor, the clever adult. Try to talk with children as to the good friend. So will respect most simply himself and the interlocutor. Track the manners to speak in the third party, for example: "Mom sees all, mom hears everything".
It is necessary to try to develop respect for own space at the kid. If he has the territory where it is felt is protected and is inviolable where nobody disturbs it, then he better perceives your right for own autonomy. Show great respect for the child's space more often. For example, you are knocked entering its room.
Give it an opportunity for the choice — though in trifles (what I go to eat what to dress, than to play, etc.). But the main thing is to believe that the child also actually is a personality sovereign and has with you general rights.