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I do not agree with the current trend of validating every single feeling a child might have. I think it leads to entitled adults who think that their feelings are above everything else, adults who think they have the right not to be hurt or offended, ever.
There's a difference between acknowledging a child's feelings ("I understand how you feel and why you feel like that"), and telling them it is OK to feel like that when it isn't.
It is OK to feel hurt when someone insulted you. It is OK to feel sad when your friend moves to another town. It is OK to feel anger when you see someone abusing somebody else.
It is not OK to feel envy of your friends toys. It is not OK to feel rage because you didn't get cake for dessert. And it is not OK to feel apathy because you had to write your name 5 times back at school.
Sure, it's not easy for a child to discern when it is OK to feel some way, and when it isn't. That's why we parents are here: to help them learn.
What about the "bad" feelings of the child? Should they be mirrored or not? What do studies say in this regard?