Is it permissible for a father to take his children's money, even if that causes harm to the children?

The majority of scholars hold that a father is not entitled to take from his son's wealth except in case of need. However, the Hanbalis permit this even if the father is not in need, provided he does not cause significant harm to his son's wealth or give it to another son. Accordingly, your husband's father is not entitled to take from his wealth what would harm him or prevent him from spending on himself and his family. Your husband has the right to demand his money and to refrain from giving his father what he does not deserve. This is not considered disobedience (ʿuqūq) if he honors him, treats him well, and obeys him in what is good. It is preferable for him to give his father what does not harm him, and you should help your husband honor his parents. It is not permissible for you to cut ties with your husband's father for more than three days without a valid reason.