Is the Aqiqah for a child Fard, and if so, if the parents did not do this obligation at the birth of the child, do they have to do it at a possible time? What can a mother do who did not know due to ignorance that she had to do Aqiqah for a girl and not only for a boy? (The children are now about 30 years old and the mother has the means to do Aqiqah)If Aqiqah was done for a boy with one sheep only, do the parents have to kill a second sheep when it becomes financially possible?

Praise be to Allah, the Lord of the Worlds; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
The correct opinion is that the Aqiqah is a (Sunnah Muakkadah) confirmed Sunnah not an obligation. According to Sunnah Aqiqah (slaughtering sheep) should be performed on the seventh day after a child birth, if one can not do it on that day then he may perform it on the fourteenth day or on the twenty-first day. This rule is based on the Hadith reported on the authority of Buraidah who said, that the Prophet said: "Aqiqah should be performed on the seventh day or on the fourteenth day or on twenty-first day after birth of a child". [al-Baihaqi] If one can not slaughter sheep as Aqiqah in the above prescribed three times for any reason he may slaughter it at any time convenient to him. If the parents or guardian of a child did not make his Aqiqah then he himself can do it whenever he is able to do it. If the guardian of a child is capable of slaughtering two sheep for a male child and one sheep for a female child he should do so. If he can not slaughter two sheep for his baby boy then he may slaughter one sheep on the seventh day. Then it is desirable to slaughter the other one on fourteenth day or twenty-first day or he may slaughter it at any other time. We say this because the Ahadith, which prove slaughtering two sheep, are stronger than those which prove slaughtering one sheep, as explained by Imam Ibn al-Qayyim. Thereupon, we conclude that it is better for one who had slaughtered one sheep for his baby boy to slaughter another sheep any time. For more details, please refer to Fatwa No.: 81462 and 81275.
Allah knows best.