I made a vow to fast if Allah would provide my son with a child. I observed the fast although my son has not yet had a child. Will I be required to fast again if my son is blessed with a child?

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

The fast that you observed before the realization of the condition that you made, which is having a child, does not free you of liability to fast when the condition will be realized. Discussing the types of vows in Bada’i‘ As-Sana’i‘, Al-Kasani said:

"If  a vow is conditional, such as saying 'If Allaah heals a sick person or if so-and-so comes back, then, by Allaah, I will fast a month, perform two Rak‘ahs (units of prayer), give a dirham as a charity, and so on, the fulfillment becomes due once the condition is realized. Scholars unanimously agree that fulfillment is not due before the condition is realized. Fulfilling a vow before the condition is realized is considered a voluntary act of worship."

Consequently, you are required to fast again if Allaah provides your son with a child. The fast that you observed before does not absolve you of liability.  

Allaah Knows best.