Child born after visa issued

Hi Faisal and welcome to the Forum.

Not sure of the process, but, assuming you and your wife have Pakistani nationality, your child (and any other children you have while having Pakistani nationality) will be Pakistani, there is no right to Spanish citizenship by being born in Spain (and many other countries); only children of Spanish citizens, or misplaced nationals inherit Spanish nationality by being born there (the term is "Jus sanguinis").  There is one exception, that being if Pakistan does not recognise the birth of your child abroad, thus denying them any nationality.

I suggest you contact the Spanish embassy/consulate where your wife has applied for her visa for details of the process you need to follow.

Hope this helps.

Thank you.