Should fatherhood be referred to by the Kunya of the eldest daughter or by the Kunya of the youngest son? And does this imply a preference for males over females?

It is permissible for a man or a woman to be known by the kunya of one of their children, male or female, whether the eldest or the youngest, or by someone other than their children. Some Companions were known by the kunya of their daughters, such as Tamim ibn Aws al-Dari by Abu Ruqayyah, and As'ad ibn Zurarah by Abu Umamah. Abu Bakr and Aisha were known by kunyas not derived from their children. Being known by the kunya of one's children does not imply preferring them over others.