Is it permissible to keep the children’s money saved in the post office, even though the mother knows that its interest is forbidden, due to the unavailability of a close alternative, and the father’s approval to use the interest for the children’s urgent needs? And what should the mother do with the money to avoid the forbidden interest?

Depositing money in the post office or any other institution with interest for the depositor is precisely usury (riba), and it is a declaration of war against Allah Almighty, bearing immense danger and sin, as Allah Almighty says: "O you who have believed, fear Allah and give up what remains of usury, if you should be believers. And if you do not, then be informed of a war from Allah and His Messenger. But if you repent, you may have your principal - [thus] you do no wrong, nor are you wronged." [Quran 2:278-279]. It is also stated in the Hadith that usury has seventy-three doors, the least of which is like a man having intercourse with his mother, and one dirham of usury is more severe in the sight of Allah than thirty-six acts of fornication. And Allah has cursed the one who consumes usury, the one who feeds it, the one who writes it down, and its two witnesses. It is not permissible to practice usury in obedience to a created being, because there is no obedience to a created being in disobedience to the Creator. The money must be withdrawn from the usurious institution, and all usurious interest must be disposed of by spending it on the needy and charitable causes. It is permissible to spend it on one's spouse and children if they are poor.