Objection 1. It seems that a man ought, out of charity, to love his children more than his father. For we ought to love those more to whom we are more bound to do good. Now we are more bound to do good to our children than to our parents, since the Apostle says (2 Corinthians 12:14): "Neither ought the children to lay up for the parents, but the parents for the children." Therefore a man ought to love his children more than his parents.
Objection 2. Further, grace perfects nature. But parents naturally love their children more than these love them, as the Philosopher states (Ethic. viii, 12). Therefore a man ought to love his children more than his parents.
Objection 3. Further, man's affections are conformed to God by charity. But God loves His children more than they love Him. Therefore we also ought to love our children more than our parents.
On the contrary, Ambrose [Origen, Hom. ii in Cant.] says: "We ought to love God first, then our parents, then our children, and lastly those of our household."
I answer that, As stated above (4, ad 1; 7), the degrees of love may be measured from two standpoints. First, from that of the object. On this respect the better a thing is, and the more like to God, the more is it to be loved: and in this way a man ought to love his father more than his children, because, to wit, he loves his father as his principle, in which respect he is a more exalted good and more like God.
Secondly, the degrees of love may be measured from the standpoint of the lover, and in this respect a man loves more that which is more closely connected with him, in which way a man's children are more lovable to him than his father, as the Philosopher states (Ethic. viii). First, because parents love their children as being part of themselves, whereas the father is not part of his son, so that the love of a father for his children, is more like a man's love for himself. Secondly, because parents know better that so and so is their child than vice versa. Thirdly, because children are nearer to their parents, as being part of them, than their parents are to them to whom they stand in the relation of a principle. Fourthly, because parents have loved longer, for the father begins to love his child at once, whereas the child begins to love his father after a lapse of time; and the longer love lasts, the stronger it is, according to Sirach 9:14: "Forsake not an old friend, for the new will not be like to him."
Reply to Objection 1. The debt due to a principle is submission of respect and honor, whereas that due to the effect is one of influence and care. Hence the duty of children to their parents consists chiefly in honor: while that of parents to their children is especially one of care.
Reply to Objection 2. It is natural for a man as father to love his children more, if we consider them as closely connected with him: but if we consider which is the more exalted good, the son naturally loves his father more.
Reply to Objection 3. As Augustine says (De Doctr. Christ. i, 32), God loves us for our good and for His honor. Wherefore since our father is related to us as principle, even as God is, it belongs properly to the father to receive honor from his children, and to the children to be provided by their parents with what is good for them. Nevertheless in cases of necessity the child is bound out of the favors received to provide for his parents before all.
Source: Summa Theologica (New Advent)