Is it permissible for a wife to take money from her husband without his knowledge and save it for the future needs of her children, knowing that he is wasteful and spends money on unlawful matters?

If the husband spends on his wife and children the obligatory maintenance, it is not permissible for the wife to take anything from his money without his knowledge, even if he is extravagant or spends money on unlawful things, or for the purpose of saving. For she has no authority over his money as long as he spends according to what is customary. As for his disobedience, the wife should advise him and remind him of Allah, not seize his money. And the maintenance for the future is not yet an obligation on him to be held accountable for.