Because I'm pretty sure both Scripture and Church Sacred Tradition make it clear that it is in Baptism that we become children or sons of God. We may all be made in the image and likeness of God but that doesn't necessarily mean we are all children of God. Once again, it is in accepting His Son that we become part of God's Family. Atheists (and others ) have not done that (yet).
You make good points, but my response was more general. I meant children to mean that we are all creations of the Father.
When I think of atheists I know, they are very different. One lacks the gift of faith, while another appears prideful and presumptuous.
The first does good for good sake, and tries to live (unknowingly) according to the natural law. The second does good, but makes the point of his atheism.
In viewing these two cases, I can see how God is trying to call both to himself, though I can see Him more generous to first. Think invincible ignorance vs. pride.