Richard Warshak, the author of Divorce Poison, has written an excellent article explaining parental alienation.

Parental Alienation: The Psychology of Fractured Parent–Child Relationships - Richard Warshak
Parental Alienation in High-Conflict Divorce

In the article, he states that any parent that enlists children as allies in a battle against the other parent is engaging in tactics to alienate children against the “target” parent.

Children who are influenced show classic behaviors that you can identify.  Warshak explains these to give the targeted parent signs to look for to intervene as early as possible.  This is critical.  Read more here.