Sadly, it is often the children who are the victims of the unresolved issues between their parents. Their parent's anger, even hatred, toward their former partner spills over onto them. Even some of the best-intentioned parents can inadvertently pull on their kids to be allies in their ongoing conflict with the other parent. Being in the middle of the fight between the two parents they love can tear the kids emotionally in two.
Keeping kids out of the middle of their parents' fight is essential if they are to feel safe when with either parent. Kids who are protected from their parents' negative opinions about each other are more likely to be able to trust and love in their own relationships someday.
Common ways that parents put their kids in the middle can be summed up in what I'm calling the "Poison Ps".
Pumping: The parent pumps the kids for information about their other parent's life. The parent insists that the kids share what they know about how money is being used, how the other parent is spending time, and whether there is a new romance. The parent is either trying to collect ammunition for their ongoing fight or they is trying to reassure themselves that getting a divorce was the right thing to do. The kids want to please but they don't want to "tattle" on their other parent. It's an awful bind.
Poisoning: The parent misses no opportunity to tell the kids how awful their other parent was and is.The parent hopes to ensure the children's loyalty by making the other "side" look as bad as possible. Kids, being kids, want to love both of their parents. Trying to make them afraid of a parent who isn't dangerous - no matter how difficult they were in your relationship - isn't fair.
Privileging: I don't know if this is really a word but it is a behavior. One parent attempts to win the kids' alliance by giving them privileges or relaxing basic rules just to make life harder for the other parent. He or she lets the kids get away with not doing chores or neglecting homework, or lets them watch video games all night or lets them eat what they please when they please. When the other parent tries to get the kids to behave or complete chores, the kids, being kids, are bound to say "Mom/dad doesn't make me do that! Why should I have to do it here?" To kids, the parent who is actually doing the better parenting can look like the "bad guy".
Passing messages: Divorced parents who can't stand to talk to each other sometimes use the kids to pass information back and forth. Children often don't remember accurately or avoid conflict by "forgetting" to mention it. This can lead to parents blaming each other for bad communication. Worse, kids often get the brunt of parent upset when the parent doesn't like the message.
The challenge for parents who divorced contentiously is to love the children more than they hate each other. That means dealing with each other directly, honestly and reasonably instead of putting the kids in the middle of their fight. It takes all the maturity that parents can muster to avoid the poison Ps out of love for their kids.