All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
First of all, it should be mentioned that in Islam the parents have a great right on their children as we have already clarified in Fataawa 87019 and 84942. Therefore, as long as the children can help them in what is not a disobedience to Allaah, they should do so as a way of being kind and dutiful to them and as a way of seeking their pleasure.
On the other hand, the parents should not financially overburden their children by asking them for too much money especially if they are not in need. Rather, their children are not obliged to obey them if they ask them for money while they are harmed by giving it to them. In such a case, it is more appropriate for the children to try to please their parents with nice words and apologize to them in a very nice and gentle manner. Moreover, it is permissible for the children to use Tawriyah (i.e. saying something which has more than one meaning and intending a meaning different from what the listener is likely to understand) by informing them for instance that they only possess what is enough to meet their necessities or that they do not even possess what to meet their necessities. So, one may use Tawriyah instead of lying. For more benefit, please refer to Fatwa 86021.
Furthermore, it is not permissible to help the parents in what is a disobedience to Allaah as Allaah Says (what means): {Help you one another in virtue, righteousness and piety but do not help one another in sin and transgression. And fear Allaah. Verily, Allaah is Severe in punishment.}[Quran 5:2] Besides, Ibn Mas’ood narrated that the Prophet said: "There is no obedience to a creature in what is a disobedience to Allaah.” [Ahmad]
Therefore, if the mother asks her children for money to use it in what is a disobedience to Allaah, it is not permissible for her children to help her in any impermissible matters, among which is her travel without a Mahram.
As regards helping the father in repaying the instalments of his Riba-based loan, then there are some details about this. If the father had repented from dealing with Riba, then it is permissible for the children to help him in this, as doing so is considered as being kind to him. However, if he did not repent and it is predominantly believed that if one helps him in repaying his loan it may be a help for him or an encouragement for him to borrow money with Riba again, then it is not permissible to help him pursuant to the previous verse. In case one does not know the matter (i.e. the father’s intention to borrow money with Riba again), then it is acceptable to help him in repaying his debts.
Finally, it should be noted that it is not an obligation, nay it is not permissible, for a person who borrowed money with Riba to repay any interest more than the capital money which he had borrowed unless he is forced to do so. For more benefit, please refer to Fatwa 86521.
Allaah Knows best.