Is the Sadaqat al-Fitr obligatory upon the wife or the husband on behalf of the children, and what is the difference between Sadaqat al-Fitr and Zakat al-Mal?

Zakat al-Fitr is obligatory upon every Muslim who possesses surplus wealth beyond their sustenance and the sustenance of their dependents for the day and night of Eid. It is obligatory for both the young and the old. This is based on the Hadith narrated by Ibn Umar (may Allah be pleased with them both), that the Messenger of Allah (peace and blessings be upon him) "enjoined Zakat al-Fitr, a saa' of dates or a saa' of barley, upon the slave and the free, the male and the female, the young and the old among the Muslims, and he commanded that it be paid before the people go out for prayer." The husband must pay it on behalf of his wife and children because it is part of maintenance, unless he is unable, in which case the wife may pay it voluntarily. "Fitrah" refers to the charity of Fitr which becomes obligatory upon the setting of the sun on the night of Eid al-Fitr. As for "Zakat," it is the specific amount that is taken from wealth once it reaches the nisab (threshold) and a full year has passed over it.