Is it permissible to pay off the debt of a son who is in need when one has other children?
Question

                I know that it is obligatory to treat one's children fairly, but one of my children is poor and has debts. Is it permissible for me to pay off some of his debts from my own money?
Praise be to Allah.It is obligatory to treat 
    one's children fairly. See Question no. 22169. 
It is obligatory to treat 
    children equally in gift-giving and it is forbidden to single some out for 
    gifts or to show preferences, unless there is a compelling reason for doing 
    so. 
If there is a reason to 
    show preference to one or to single one out, then it is acceptable to do so, 
    such as if one of them is sick, blind or disabled, or he has a large family, 
    or he is a student, and other reasons. So in that case it is acceptable to 
    give him preference for those reasons. 
Imaam Ahmad referred to 
    that when he said – concerning singling out some of one's children to be 
    beneficiaries of a waqf – “There is nothing wrong with that if it is because 
    of a need, but I regard it as makrooh if it is by way of showing 
    favouritism.” 
Shaykh al-Islam Ibn 
    Taymiyah said: “The hadeeth and the reports indicate that it is obligatory 
    to treat children fairly… but there are two kinds: 
1 – Those who need him to 
    spend on them in sickness and in health and so on, in which case fairness 
    means giving each of them what he needs, and it makes no difference if he 
    needs a little or a lot. 
2 – Needs which all the 
    children will have in common, such as gifts, maintenance and arranging their 
    marriages. Undoubtedly it is haraam to show preferences with regard to these 
    matters.
From these two stems a third kind, which is helping out one child for an extraordinary need, such as paying off the debt of one child, or paying compensation on his behalf (if he has caused injury to another), or paying the mahr on his behalf, or giving him the money he needs for his wife’s maintenance, and so on. Concerning whether or not he is obligated to give a similar amount to the other children is subject to further discussion.” (From al-Ikhtiyaaraat)