Théun, I have heard it mentioned that you advise parents to join forces in order to have a united front in dealing with children. What exactly do you mean by parents having a united front? Does it mean that both parents must explain things to the child in exactly the same way?
The meaning of having a united front is not so much a matter of both parents explaining things to the child in the same way, for this is not in itself important. It is in fact a good thing if parents each have their own way in explaining something to the child. What is meant by the parents having a united front is that both parents must agree on what are the child's parameters.
An example of where there is no united front is where the father may say the child must be punished for some offence, but then the mother steps in and says no, she does not think the child should be punished for that particular offense. It is okay for the parents to disagree, provided they discuss their disagreements in private amongst themselves. But they should NEVER disagree or discuss their differences in front of the child. To do so is the worst possible mistake to make with children because the child concerned will quickly enough see this as one parent taking its side. And once the child has this perception it will quickly enough learn to manipulate its parents and how to play one off against the other.
So it is absolutely important that when in the presence of the child both parents support one another. However, even WHEN there is a united front children can STILL be ingenious in how they can manipulate their parents. For example, let us say that Mom is quite a softy, but Dad is quite firm. I can promise you that it won't take Junior too long to see this for himself. And so Junior first goes to Mom and asks, "Mom can I go to the dance at the school on Saturday night?"
Mom, in not being at all sure that Dad will agree, makes the fatal mistake of replying, "I don't mind if you go to the dance, but let us first check with Dad when he comes home this evening. Okay?"
This is all that Junior needs in order to start playing Mom off against Dad. So when Dad comes home that evening and refuses to let Junior attend the dance, Junior is going to sulk and blame his father for being unreasonable. He will, of course, go and complain about this to Mom, but what is Mom now supposed to do? She has set herself a trap, in that if she agrees with Junior she will be taking sides with her son against his father. And if she disagrees, Junior is going to blame HER for being unfair in that she is now taking sides with his father! If, on the other hand, Mom says, "Leave it to me darling. I will speak to Dad later," she has just stepped into an even worse trap, for if she manages to convince her husband to let Junior attend the dance, Junior will never again take his father seriously on anything his father says to him. If, on the other hand, Dad stands firm, Junior will start to see his mother as being weak and ineffectual, and will therefore lose his respect for her.