Further information I've received is, specific to "Parental Alienation and Visitation Refusal Behaviour".
It seems that a spouse who does not encourage the children to attend visits could be unknowingly participating in Parental Alienation. And a kid who is basing their refusal to visit based on emotions could be found to have unwarranted "Parental Alignment and Rejection".
It is really interesting stuff, even though it does not help the situation. Legal advice and counselling for the kids seems to be the most recommended procedure to get through this.
