http://revach.net/article.php?id=2375

Section: Taryag Mitzvos   Category: Minchas Chinuch
Piru URivu If Your Children Are Not Yet Child Bearing Age
To fulfill the Mitzva of Piru U’Rivu a man must have both a son and a daughter.  The Shulchan Aruch paskens (EH 1:5) that these two children must be able to have children of their own.  If they cannot have children then they are not counted towards the man’s Chiyuv of Piru U’Rivu.  If one has fulfilled his mitzva he has no Chiyuv of Piru U’Rivu to actively pursue having more children.

If your child is still a Katan and has not reached child bearing age you are still considered to have fulfilled your Mitzva according to the Rema (1:6).  However the Minchas Chinuch points out that when a boy reaches Bar Mitzva he is assumed to have reached puberty (Shtei Sa’aros).  However if it is revealed that he doesn’t have the required two hairs, it is then a Safek if he had them and they subsequently fell out or he never got them. Therefore he is safek “Sris” that cannot bear children.  In this case the father would be once again required to actively pursue the mitzva of Piru U’Rivu.

The above is taken from the Minchas Chinuch and is not in anyway P’sak Halacha.  Furthermore it is the author’s understanding of the Minchas Chinuch and may not being completely accurate.