I voted "No babies are atheist".
The word atheism describes more than one position towards the idea of god. The particular belief that there is no god(s), the rejection of the idea (on a variety of grounds), and the mere absence of belief in a god(s). Given that the word describes a worldview, a position, and as mentioned above differing ones at that, it's unhelpful to include people who have no knowledge or ability to embrace or reject a worldview in the first place.
Technically, including them can pass. But it's unhelpful because that's not what the word strikes me to be aimed towards. It's describing an embraced position. To include babies would also include everything and anything that does not believe in a god(s). This means that trees are atheists, insects are atheists, sand is atheist, earth is atheist, the sky's atheist etc... I think it's certainly reasonable to conclude that this is not what the word is supposed to be addressing.
The approach of including babies is similar to that of some religions which propose that babies by default fall under it's label because they're 'in accordance with god's creation' or anything to that effect. Neither positions is helpful because it doesn't actually say anything about the baby.