The Church commonly uses the phrase, “children of God,” in reference to those who have been baptized. For example, almost every time the phrase is used in the Catechism of the Catholic Church, it implies that one becomes a “child of God” after baptism. Such usage emphasizes that through baptism we enter into the covenantal family of God (see CCC 546, 1243, 1244, 1250, 1676).
Now, paragraph 1676 does use the phrase, “child of God,” in reference to every human being having the same origin, God.
So, while the phrase can be used metaphorically in reference to humans being created by God—we are like his children in that we come from him like a child comes from his father—it is more proper to use it in reference to those who actually become children of God by adoption through baptism.
I think it is safe to say that Pope Francis was using it in reference to all humans having the same origin, God and thus trying to emphasize our common dignity. Could he have qualified the statement in order not to confuse people and take away from the dignity of baptized Christians? That is a question of prudence that is subject to debate.
Here is an answer to the same question that may be of help:
forums.catholic-questions.org/showthread.php?t=994424