What should I do when my father demands to see my children, even though he has insulted me and my wife and expelled us from his house?

We advise you to be patient with your father and supplicate for him, and to strive to reconcile with him. Honoring one's parents is obligatory even if they wrong and oppress, and part of honoring him is to obey him in his request to see the children. This is because obeying him in that is an obedience in what is permissible, and it is not permissible to prevent them from seeing their grandfather, lest you be disobedient and cause a severance of kinship ties. If you fear for them, you can accompany them when they visit. Perhaps that will mend the situation between you, in fulfillment of Allah's saying: "Repel evil with that which is better, and then verily he, between whom and you was enmity, (will become) as though he was a close friend."