It's not clear that there is any sense in which we created beings can "know" God's plan—for the universe, or for any being or group of beings. What we can know generally is that God wills good, and salvation, for all beings:
God, who "dwells in unapproachable light," [cf. 1 Timothy 6:16] wants to communicate his own divine life to the men he freely created, in order to adopt them as his sons in his only-begotten Son.
(Catechism of the Catholic Church, paragraph 52; emphasis added)
It is not we, but God, who creates each human being, body and soul. Having created each being, it seems apparent that He certainly wished for them to be. Whether God planned for them to be born this way?—now we're getting into questions of foreknowledge and God's interaction with time, always a rough area given that God is outside of time. The very least, and perhaps the most, that we can say, is that children are a gift from God, regardless of the circumstances in which they are conceived, born, and raised:
Children are really the supreme gift of marriage and contribute very substantially to the welfare of their parents.
(Gaudium et Spes, section 50)
As far as their legal status (by which I assume you mean their status in canon law, as their civil status might vary by jurisdiction) and spiritual status, canon law is very clear on this:
The children conceived or born of a valid or putative marriage [that is, a marriage which is thought to be valid by at least one of the parties] are legitimate.
(Code of Canon Law, Canon 1137; emphasis added)
Both parents have an obligation to care for the children:
[If a decree of nullity is issued] the parties are to be reminded of the moral and even civil obligations which may bind them ... toward their children to furnish support and education.
The Catechism has a section on the duties of parents towards their children (paragraphs 2221–31). Nowhere in that text is it stated, or even implied, that parents do not have these responsibilities if they did not have a valid marriage.
In short, though we can't know, in any sense, what God's plan is with respect to the children of these people, the Church treats them in all senses the same as children of a valid marriage.