Is it permissible to give each member of the poor family—both adults and children—one and a half kilograms of food to expiate an oath, and would that suffice as expiation?

There is no objection to giving the expiation for an oath (kaffarat al-yamin) to a single family if they consist of ten needy individuals, whether young or old, male or female, as the consideration is their being needy (masakeen). If you give each needy person one mudd of food (approximately 650 or 750 grams) from the average of what you feed your own families, then you have expiated for your oath. Allah (the Most High) says: "So its expiation is the feeding of ten needy people from the average of what you feed your [own] families" (Al-Ma'idah: 89). If you give each needy person one and a half kilograms, then you have fulfilled your obligation, and the excess is charity.