I would agree that a letter, written from the heart and given to each child personally, might be a good approach. Dad ought to consider dealing with the children's emotions in those letters, and NOT rehash the events surrounding the divorce.
The use of force through the courts will likely further alienate the children.
They're, all of them, old enough to have developed their own perception of the events surrounding their parent's divorce.....and to have assigned blame. Their mother may, or may not, have influenced them.
Generally speaking, I think it's the knee-jerk reaction of divorced fathers to think that Mom is unduly influencing the children....but that doesn't give the kids enough credit in having developed their own feelings, right or wrong. And worse, it doesn't allow Dad to deal with those feelings quite as effectively, since he believes them to be
crafted by someone else.
It doesn't really matter what the real truth is. Perception itself IS the truth.... insomuch as THAT is what must be dealt with.
He should continue to express his desire for visitation, and should take advantage of whatever in-roads he has with his children by seeing them without your presence. (Unfortunately for you, and whatever plans the two of you might have previously made for 'one-big-happy-family', you are bound to be caught up in the children's negative perceptions.)
Some of those children may indeed come around in time. Some may never truly forgive him. All will likely find
tolerance for him eventually, if he's willing to excersize patience now. It's sad, but it's the nature of people. Some are just more forgiving than others.
Being a "right-fighter" will not help Dad at all. He may feel that the divorce was in no way his fault. Maybe he's even right about that, who knows?
The kids aren't capable yet of seeing that aspect of it. What the kids see is that Dad allowed their family to be torn apart. It was his job to protect them and to keep that from happening....no matter what. He is the provider and the
defender of the family. It won't matter who was right and who was wrong. In this, he has failed.
Time will be the cure. Young people need
time to grow and develop maturity in their thinking. Their feelings are genuine enough, right or wrong. But the concepts they are forced to deal with are adult ones. The kids just aren't mature enough to truly understand yet.
Persistance and patience. Both in GREAT quantity will pay off in the end.