I’m one of those people who believes on giving people honest, upfront information. It might help the situation to sit down with your children and explain why you and grandpa don’t talk. Not to vilify him, or forbid them to interact with him, but to explain why you don’t. That might help, or at least give them forewarning because if he was like that with you, than you know, if he gets the chance, he will use your children to make your family as dysfunctional as his own.