Remarrying when there are adult children is an interesting situation. Where is it written, that you are not supposed to be a grandparent if you are not related by blood. That's what some ex-spouses thinks. There seems to be an unwritten rule that the grandchildren belong to them and no one else is allowed to love them.
Children don' t understand divorce and that life goes on even after that.
They only understand that someone loves them unconditionally. Children can't have too many grandparents that love them. Classic example, birthday parties for the grandchildren. In attendance, grandparents on both sides, uncles and aunts, cousins and children's friends. Ex-wife and grandpa's current wife are content to sit on opposite sides of the room.