What is the ruling of Islamic law regarding a man marrying a second wife if his first wife causes many problems and threatens divorce (khulʿ), and the husband fears for the future of his children in the event of separation? Would a second marriage protect the man from evils and temptations?

It is permissible to marry another woman; this is a right guaranteed by Islamic law. Allah Almighty says: "Then marry those that please you of [other] women, two or three or four. But if you fear that you will not be just, then [marry only] one or those your right hand possesses. That is more suitable that you may not incline [to injustice]." If you fear separation for your children, then be patient and try to mend your relationship with your wife. However, if you fear falling into sin, then hasten to marry if you are able to bear its responsibilities, and Allah will take care of your children through your piety. It is permissible to conceal the marriage from the first wife because it is not obligatory to disclose it.