Praise be to Allah.
In principle, it is not permissible for the man to give the zakaah of his wealth or his zakat al-fitr or his expiation to his daughter, because if she is poor and he has wealth, he is obliged to spend on her.
But some of the scholars made an exception from that in two cases:
The first is that if he gives it to her for a reason other than poverty, such as if she is in debt, so he gives her something to pay off her debt, because it is not obligatory for the father to pay off the debts of his children.
The second is if he is unable to spend on her maintenance.
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said in al-Ikhtiyaaraat (p. 104): It is permissible to give zakaah to parents and grandparents, no matter how far the line of ascent reaches, and to children and grandchildren, no matter how far the line of descent reaches, if they are poor and he is unable to spend on their maintenance. The same applies if they are in debt, or are slaves with contracts of manumission, or are wayfarers. If the mother is poor, and she has little children who own wealth, but her taking from their wealth for her maintenance will be detrimental to them, she may be given something from their zakaah. End quote.
If the husband is unable to spend on his wife’s maintenance, does it become obligatory for the father to spend on her maintenance? Regarding that there are two scholarly views. The Maalikis are of the view that it is obligatory for him.
Al-Kharashi al-Maaliki said in his Sharh ‘ala Mukhtasar Khaleel (4/204): [His duty to spend on her] is not waived if she marries a poor man.
That is, the duty of spending on the mother is not waived for the son if the mother marries a poor man, or a rich man who then becomes poor, because the husband’s presence makes no difference. The same applies to anyone who is obliged to spend on a woman; that duty is not waived if she marries a poor man.
But if she marries a rich man, his duty of spending on her maintenance is waived, so long as there is no circumstantial evidence to the contrary.
The daughter is the same as the mother in that regard.
If the husband is able to spend on some of her maintenance, the son or father should spend on the rest. End quote.
The Shaafa‘is are of the view that this is not obligatory for him.
Al-Khateeb ash-Sharbeeni ash-Shaafa‘i said in Mughni al-Muhtaaj (5/185): If she [that is, the mother or the daughter] gets married, the duty of spending on her maintenance is waived once the marriage contract is done, even if the husband is not well off, until the marriage is annulled, lest there be two people spending on her maintenance. End quote.
Assuming that spending on her maintenance is obligatory for the father because the husband is not well off, then according to the view of Shaykh al-Islam [Ibn Taymiyah], it is permissible for a father to give his zakaah to his daughter if he is unable to spend on her, but in that case it is better to give it to her husband, so as to avoid a matter concerning which the scholars differ.
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) was asked: Is it valid to give zakaah to a married daughter who is in need?
He replied: With regard to anyone who meets the criteria of being entitled to receive zakaah, it is permissible to give zakaah to him.
Based on that, if the father is not able to spend on his daughter and her children, then he may give zakaah to her. But what is better and more prudent, is to give it to her husband. End quote.
He was also asked: Is it permissible for me to give some of my zakaah to my married daughters, knowing that they are poor?
He replied: The scholars stated that a man should not give his zakaah to his offspring, or to his father and grandfathers, or to his mother and grandmothers; in other words, he should not give it to his ascendants or his descendants. This applies if he is giving it to them in order to meet their basic needs. But if they owe debts that they cannot repay, then it is permissible for him to give zakaah to them, because he is not obliged to pay off their debts, hence giving his zakaah to them in this case is not saving his wealth.
Summary of the answer: Regarding this man who has married daughters whose husbands are poor, if he does not have enough money to spend on their maintenance, there is nothing wrong with him giving his zakaah to them, but he may give the money to their husbands, because they are responsible for spending on their maintenance. There is nothing wrong with that in any case." (Majmoo‘ Fataawa Ibn ‘Uthaymeen 18/426).
And Allah knows best.