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