What is the ruling on marriage without a legal contract, and is it considered fornication? And what is the ruling on children visiting their father in a house where the relationship between the father and the woman is considered fornication?

If it is confirmed that a man is living with a woman without a valid marriage contract, he must repent and separate from her immediately until he contracts a valid marriage with her. As for the children, they must obey their father, honor him, treat him kindly, and visit him, even if he is negligent or disobedient, while also advising him and reminding him of Allah. Their entering the house to visit their father and treat him kindly is not forbidden, and they can denounce the wrongdoing according to their ability.