This is an interesting question.
It is possible to say that everyone is a child of God because God created them. So they are his children just as our own children are our children even if they are deeply in sin and lost souls now. In this sense God’s children could be in hell, simply because God had created them.
Men can become children of God though in quite another sense–as the Gospel of Saint John explains, to all who received Christ, “who believed in his name, he gave power to become children of God” (John 1:12). This seems to speak of being so through God’s grace. But in any real sense, lost souls do not receive Christ, and so, it would seen, no longer are children of God in this second sense.
(Looking back on the other posts, I see Mary_Ellen also said a similar thing.)