NLW, when I first read the original interaction, my jaw hit the floor. And based on his message afterwords, he obviously is completely clueless and continues to try to push and enforce his will on everyone else.
There is nothing you or your kids could say to him that will help him understand your perspectives. He simply will not hear it and, like many, will simply use the opportunity to remind you or the kids how wrong you all are and how right and justified he is.
It really is up to him to figure out why his kids are so upset with him. That will require him to eat a bowl of humility and bring his empathy to the plate, of which again, like other WAS / MLCers, he has none.
If he wants to take this to court, the kids will have ample opportunity to speak their case to counsellors who will be able to present their position to the courts.
If you can, give the kids the opportunity to speak to counsellors NOW, to help them work through this.
And put that letter away in the file as a matter of record and move forward without responding.
