Must I fast again if Allah blesses me with a child after I had vowed to fast for offspring that Allah had not granted me?

If a person fasts a conditional vow – such as the birth of a child – before the condition is met, this fast does not fulfill the vow, and they must fast again when the condition is met. This is what Al-Kasani stipulated in "Bada’i’ al-Sana’i’", explaining that the time for fulfilling a conditional vow is the time the condition is met, and whatever was done before that is considered a supererogatory act.