It's a very good question. Not uncommon after divorce. I think the best thing that can be done is documenting what you're experiencing, what the kids are experiencing and if it continues, I think then the children need to be able to have that voice.
Perhaps, if it is really significant, to get a caseworker involved to listen to what the children are experiencing with the other parent. Now, again, I don't know the extent of that, being bullied but I think if they're telling you those things, then the documentation of it is probably one of your best strategies right now.
Now that being said, I think it is important that it's not just you that they're telling. It would be helpful to have someone else that they could share what they're experiencing. Maybe it's a school counselor, a friend, a family member, a religious leader, coach. Again, I know this might sound a little bit strange, but it needs to not just be you because then it's your word against his word.
I think if your children are sharing with someone else, then there's more significance to it. Not that your word won't matter, but it's simply you got divorced. It's him versus her and the children get in the middle.
I wonder why they can't talk with somebody else, or they're afraid to, they don't want to get their Dad in trouble? These are questions that I would want to know.
If these things continue the best idea might be to take your children to a professional therapist, so they can talk openly about what they're experiencing. With therapist support you have additional documentation. If things continue to escalate, then you can take that information and say, "look, the children don't want to visit because of these reasons."
But your children, at least it's been my experience, are pretty protective of their parents. My hope is that, in this case, if it's something that they can talk about with somebody, a professional, it could maybe create some change.
It's absolutely important that you do take a stand. We don't ignore things like this because the children feel like they've lost a voice and then they'll pretend like everything is okay. I want your children to have a voice, to be able to be authentic and be real. If at all possible I'd like it if they could actually tell their father that they don't want to be treated that way and create their boundaries. It's a skill to be able to stand up for yourself and to protect yourself.
I wish you guys the best. This is not an easy time. May you guys be blessed as you go through this.
Thanks for your question.