We see it happening far too often, divorced parents trying to turn the children against the other parent, or the other parent's choice in a new partner. It even happens in families where the parents are not divorced or even separated.
I can tell you from experience, this does nothing but hurt the child or children, and in the end it will cause them to resent you, not the other parent!
I didn't grow up in a conventional two parent family. I was born to a single mother, but spent much of my time in another home. I call it a foster home to make the explanation easier, although it was not court mandated. My "foster" mom was an older woman, divorced twice, who'd already raised two boys before I was born.
Although my biological mother and foster mother were friends before I was born, their friendship dissipated not long after, however they remained in contact for my sake. I spent weekends and school vacations with my foster mom, and school days with my real mother. As I grew up the two of them actually began hating each other, and I think each was jealous of my affection for the other.
Although she meant well, my foster mother would never stop trying to convince me that I should hate my real mother. My real mother was young, immature, and irresponsible. I was often told horrid stories of things that my mother did before I was born, and when I was too young to remember. She was a completely frustrating individual, a free loader, and above all, selfish. The things my foster mother told me were not lies. She used horrible words to describe my mother, and at one point I even went along with her and started using those words about her myself.
I grew tired of hearing about all my mother's faults. I knew she wasn't perfect, but deep in my heart I loved her anyways. I went along with my foster mom just to make her happy, basically so she would stop, but it hurt me. I was sad every time I heard bad words about my mother. I often cried myself to sleep. I always wondered how my foster mom, who I also loved dearly, could be so cruel to the other woman I loved.
As I grew older I learned to resent my foster mother. Her feelings towards my mother were tearing us apart. But at the same time I was beginning to see she was right. When I was twelve years old I chose to move in full time with my foster mom, but my resentment towards her only grew worse. At sixteen I changed my mind and moved back home full time with my mother. After that I didn't speak to my foster mom for four years.
Living with my mom full time was all I needed to really open my eyes. I really did grow to hate her more and more everyday. I had many struggles throughout my early adult life. I lived in shelters a few times, and stayed with other friends, all in hopes of escaping my mother's house. None of which worked out.
I eventually did move out permanently, and then reconnected with my foster mom. Since then I had a great relationship with my foster mom until she died in February of 2007. My relationship with her was so great, I often refer to her as my real mother, as if she was the only mother I ever had. So far as my real mother goes.. I haven't spoken to her in years, and have no plans on trying to contact her. Even after all this time, I still have far too much anger in my heart towards her.
Although my story is long and complicated, the point is, parents should never bad mouth each other around their children. All that does is create confusion and resentment in the child. It is only natural for a child to feel love towards their parents, regardless of that person's faults. As the child grows older, they can learn for themselves whether or not that person deserves their love and affection. But ultimately, the choice of what to think about their own parents should be left up to the child, and not coerced by another adult.