Imam an-Nawawi, may Allah have mercy upon him, in his commentary of the Muhaźźab said:

We said that the fitrā of a child , and generations below (descendants eg grandchild, great grandchild etc) is upon his father and the rest of his parents, and grandparents. The fitrā of a father, and generations above (eg grandfather, great grandfather etc) is upon the offsprings. These are applicable with the condition of the obligation of nafqa (sustenance), i.e. The obligation of a person to discharge fitra for the other is only upon one who is responsible to provide the other’s nafqa. If nafqa is not obligatory upon that person then fitra is also not compulsory upon that person. If a young child is mūsir (well off), his fitra and nafqa is upon his own money, not upon his father or grandfather. This is the opinion of Abū Hanīfa, Muhammad, Ahmad and Ishāq. Ibnu al-Munźir narrated from some of the ‘ūlamā: that it is upon the father and if the father discharges from the wealth of the child, he sins and is liable for it.