All humans can be divided into two categories:
1) Those who hold the belief that a god exists (theist)
2) Those who don't fall into category #1 (atheist)
Since newborn babies wouldn't even be aware of the concept of a god, they don't hold a belief that a god exists. Therefore they fall into category #2 and are atheists.
A belief is a predetermined decision.
A rational decision is deliberated with an open mind, considering available facts and arguments.
A belief is formed when we identify a reliable pattern so as to provide a shortcut in calculations of thought.
A baby has to form a belief about God at some stage. Usually a person first forms a belief about God while they are still young and impressionable, so their belief is more of a reflection of the environment in which they were making the decision. A child's ability to reason is impaired by their naivety, so that they are less likely to resist and more likely to concede to an authority's point of view, even if it doesn't immediately jive with their intuition.
(Some people don't really grow out of that naivety into a leadership authority, so producing communities of ignorance).
When a child is encountering the God concept, they are also learning. Knowledge consists of spirit and information.
God is spirit, the spirit that is holy. Some of the qualities of that spirit are truth, righteousness and love. A person encountering information in such spirit will come to understand the knowledge of God enough in order to understand and agree with the concepts that The Holy Spirit teaches, thereby forming Christian beliefs. If a person is viewing the information with an unholy spirit, having chosen that perspective in order to corroborate and justify their desire for sin, they are not receiving the information in a way that is agreeable to what The Holy Spirit is conveying. Therefore they are forced to oppose either The Holy Spirit's teaching or their own perspective.
A child does not naturally have an interest in opposing The Holy Spirit's teaching, because they do not naturally have a spirit that is unholy or sinful. We are made in the image of God, and it is when we are tempted to do sin, that sin manifests in us (eg: pride, wrath, envy, greed etc).
For this reason, if a child is introduced to the knowledge of God by a person who has holy spirit, they will find the concepts agreeable (children are naturally more agreeable with truth and righteousness than grown-ups, because they haven't yet been tempted and corrupted by sin so much, nor do they have ongoing pressures of anxiety, resentment and discontentment etc that produce a constantly unholy spirit).
If a child is introduced to the concept of God in an environment that is not of holy spirit, then they are tempted to take part in the perspective of conceit, that is to say "oh nah, we don't believe in God". It is possible, however, that upon encountering an opportunity to learn at a later time, they could still have God's favour because the fact is that they didn't consciously choose to reject the knowledge of God that The Holy Spirit had conveyed to them .. but it does also depend on who they have become as a result of the course they have taken in life.