As you have some contact with a solicitor, I wonder if sending letters via this agency to be given, in person to your grand-children by the lawyer, would ensure they got them?
Just short notes saying how much you love them and think about them - nothing Dad could object to as he will surely read them as well.
Of course you may think this would make things worse.
It truly is a very sad story and I hope you can try to look after yourself and DH now so that when your grand-children do contact you (live in hope ) you are able to enjoy them.
Anger management!!! Help needed.