Mosiah 15 addresses this topic of who the seed spoken of in this verse in Isaiah are. It is not speaking of literal children, but rather all those who are adopted into his grace and mercy through the atonement:

” 10 And now I say unto you, who shall declare hisgeneration? Behold, I say unto you, that when his soul has been made an offering for sin he shall see his seed. And now what say ye? And who shall be his seed?

11 Behold I say unto you, that whosoever has heard the words of the prophets, yea, all the holy prophets who have prophesied concerning the coming of the Lord—I say unto you, that all those who have hearkened unto their words, and believed that the Lord would redeem his people, and have looked forward to that day for a remission of their sins, I say unto you, that these are his seed, or they are the heirs of the kingdom of God.

12 For these are they whose sins he has borne; these are they for whom he has died, to redeem them from their transgressions. And now, are they not his seed?

13 Yea, and are not the prophets, every one that has opened his mouth to prophesy, that has not fallen into transgression, I mean all the holy prophets ever since the world began? I say unto you that they are his seed.”