The obligation to spend on children and grandchildren
Question

Is it obligatory for a man to spend on his grandchildren?
Praise be to Allah.It is obligatory for a man to spend on his children and grandchildren.
With regard to his children, that is because Allah, may He be Exalted, says (interpretation of the meaning): {And if they breastfeed for you, then give them their payment} [At-Talaq 65:6]. This verse indicates that the father is obliged to pay for breastfeeding the child.
The Prophet (blessings and peace of Allah be upon him) said to Hind, the wife of Abu Sufyan, when she complained to him that Abu Sufyan was a stingy man: “Take – that is, from his wealth – that which will suffice you and your child on a reasonable basis.”
Therefore it is obligatory for the child’s maintenance to be spent on from the wealth of the father.
Ibn Al-Mundhir (may Allah have mercy on him) said: All of the scholars from whom we acquired knowledge are unanimously agreed that a man is obliged to spend on the maintenance of his minor children who have no wealth. (End quote)
If the father is alive and is wealthy, then it is obligatory for him alone to spend on the children, and it is not obligatory for the mother.
But if the father is poor or is deceased, then the mother is obliged to spend on her children, if she is rich and they are poor.
Ibn Qudamah (may Allah have mercy on him) said in Al-Mughni (11/373):
If the father is not well off – meaning that he is poor – then spending [on the children’s maintenance] becomes obligatory for the mother. (End quote)
With regard to grandchildren, it is obligatory to spend on them, according to the majority of scholars, because a grandson may be called a “son” or “child.”
Allah, may He be Exalted, says (interpretation of the meaning):
{Allah instructs you concerning your children: for the male, what is equal to the share of two females} [An-Nisa’ 4:11].
The word {children (Awlad)} in this verse includes the children of one’s son, according to scholarly consensus, and they are one’s grandchildren.
The Prophet (blessings and peace of Allah be upon him) said regarding Al-Hasan ibn `Ali (may Allah be pleased with them both): “This son of mine will be a leader.” (Narrated by Al-Bukhari, 2704).
Al-Hasan (may Allah be pleased with him) is the son of the daughter of the Prophet (blessings and peace of Allah be upon him), his grandson through his daughter.
The fact that a grandson is called a son means that the grandson is included in the religious texts which indicate that it is obligatory to spend on one’s children.
Shaykh Ibn `Uthaymin (may Allah have mercy on him) said in Ash-Sharh Al-Mumti` (13/498-499), in the chapter on spending on relatives:
Your ascendants are those from whom you stem, fathers and mothers.
Your descendants are those who stem from you, sons and daughters.
Then he said:
You should understand that this issue is similar to the issue of what is prohibited for marriage; there is no difference between the father’s side or the mother’s side. Ascendants and descendants are the same, whether they are on the father’s side or the mother’s side or are entitled to an allocated share (of inheritance); it is obligatory to spend on them, but that is subject to some conditions. (End quote)
In order for it to be obligatory to spend on grandchildren, it is stipulated that they should be poor and have no wealth that could suffice them, and the grandfather should be rich, because the Prophet (blessings and peace of Allah be upon him) said: “Start with yourself and give charity to yourself. If there is anything left over, then (give) to your family. If there is anything left over from your family, then (give) to your relatives.” (Narrated by Muslim, 997)