Does your parenting plan have an alternative dispute resolution section? Can you go to mediation and discuss this? The leading case in WA on visits when the kids don't want to go is Marriage of Rideout. Basically it holds that a parent can be found in contempt and fined a significant amount of money for not facilitating visits with the other parent. It is a sobering read.
Your 12 year old is becoming more independent as he grows up. But he's still a kid in the eyes of the law. Also, he's going to share with you what he thinks you want/need to hear about his time in his dad's household. When he shares some vile story about how mistreated he is at his dad's, that you let him vent, but rather than responding to your ex back through your kid, e.g. "well you tell that b--- that I said . . . ." that you let him vent, and then ask him how that makes him feel and listen to the answer you get.
Then you can discuss this with your ex when your kid is not listening - at your house, or at the ex's house. If you are convinced that the new girlfriend is abusing your child, and you can prove it without relying on child hearsay, that *might* be a basis to ask for a modification. Alternatively it might be a really expensive lesson in not pestering a court over trivial stuff from a 12-year old, no matter how mature he is.
I hope this is useful, even if it isn't what you wanted to hear. Elizabeth Powell
There's very little to add to Ms. Powell's answer, as it's thoughtful and thorough. But I often parallel this situation to taking a younger child to the dentist. He or she may not want to go, but that doesn't mean it's not in the child's best interest. Courts view a child's relationship with both parents as being in the child's best interest (as long as each parent doesn't present a threat to the well-being of a child).
Likewise, each parent is expected to affirmatively act on behalf of the child's best interest. So facilitating a relationship with the other parent is each parent's duty (again, barring any danger).
Again, sort of like taking a child to the dentist.
It's not the easiest task, especially with a mature 12-year old, but in my experiences, it's what the court will expect.
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