It's entirely relevant. If you accept that there are adult atheists (and you do, right?), then arguing that babies aren't atheists means that at some point, every adult atheist went through a transition from "not an atheist" to "atheist". It implies that this point exists even for the atheists (like me) who say they've never believed in any gods.
So... when does this point occur? Does it depend on level of cognitive development?
Does it depend on rejecting all gods? If so, is this even possible?
Does it depend on rejecting a single god? If so, is there any overlap between theism and atheism?
If you're right, then the point must be there somewhere... so where is it? How old, intelligent, knowledgeable, etc. does a child have to be before we can call them an atheist?