Well, the PROBLEM I see that your having with the idea is the way that you USE "dis".
I'm fine with all of your logical explanations of why the words mean what they mean, etc.
But let's just take a look at the reasons someone might want to advocate the position that a baby, or indeed anyone in the "default" state we've been discussing, can be deemed "atheist". The reasons are, as far as I have seen, to further or strengthen the atheist argument or position. And THIS is what I take issue with. It is COMPLETELY MEANINGLESS to say babies and the completely uninformed make the realm of "atheists" a very strong category or large group, or that the community of people who consider themselves atheist is better off for having babies and the uninformed among our ranks.
And therefore, it is completely useless to use that idea in any argument on behalf of the stance atheism.
The theists: "We've got God on our side!"
The atheists: "But we've got all the babies! Hahahahaha! Aren't we clever?"
And the answer... no. No you are not.