Since we are free to choose our actions on this earth, I have always been intrigued by the question of what might have been if I chose a different path. For example, suppose that it was God’s will for someone to have six children. Let’s say that this person instead practiced family planning (either natural or artificial), so these children were never born. Would God see to it that these children were born to different parents, or would they not be born at all?
Your question seems to assume that the six hypothetical children’s souls exist somewhere waiting for their bodies to be conceived by their parents - it doesn’t work that way. A child’s soul does not come into existence until conception at which point it is created immediately by God (cf. Catechism, 366). Therefore, if someone chooses not to conceive children, children never exist.