The Catholics on FR are pretty intent on keeping Christ and heaven to themselves.
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I have not observed that.
At any rate, to the current topic: I recall being taught in Catholic school that anyone who lives a good life is doing the will of God. Those who have never been baptized and who do not know of Christ but live as good humans gain the Baptism of Desire.
1259 For catechumens who die before their Baptism, their explicit desire to receive it, together with repentance for their sins, and charity, assures them the salvation that they were not able to receive through the sacrament.
1260 "Since Christ died for all, and since all men are in fact called to one and the same destiny, which is divine, we must hold that the Holy Spirit offers to all the possibility of being made partakers, in a way known to God, of the Paschal mystery." Every man who is ignorant of the Gospel of Christ and of his Church, but seeks the truth and does the will of God in accordance with his understanding of it, can be saved. It may be supposed that such persons would have desired Baptism explicitly if they had known its necessity.
Of course, we can not assume all other people fall into this "ignorant" category (and I would argue that most people do not in this day and age), so I still believe it is wrong to include atheists and other "unbaptized" as "children of God".