Children have the right to never hear a parent, a relative, or a friend of a parent run down or degrade the other parent:p. So what to do if this happens?

That is a really tough one to answer. I am a product of that environment and it took me a lot of years to realize that Dad was not the bastard that mom made him out to be. All you can do is to remain positive and if a family member is doing this, put a stop to it in no uncertain terms. If it is an X that is harder, but please try and talk to who ever it is and let them know that this is not healthy for the children's sake.