An infant might be said to 'know' how to breath, but our use of 'know' here is misleading. We observe an infant breathing, and assume they 'know' how to do it. But do they really know?
Knowledge is defined as information, skills or awareness gained by experience or education. It requires an awareness of relationships between experiences.
So breathing for an infant is not 'knowledge' as such. For the most part, infants are not yet aware of a relationship between these actions we collectively call 'breathing', nor are they aware of how they perform these actions, or that they do so without needing to be consciously aware of it.
An organism's life support systems operate most efficiently on a 'need to know' basis. The total knowledge required by a human being in order to 'breath' is held, not in the consciousness of the infant, but across various cells within the life support system: from air sacs in the lungs to diaphragm muscles and airways. Each of these cells has the capacity for limited functions including relationships with other cells, accomplishing what is required of it by the organism, but each cell is unaware of what we call 'breathing' as a cooperative effort on their part.
In this sense, there is no need for an infant to know they are breathing in order to breath successfully. It's only when they experience or observe a struggle to breath, or receive some other relevant education, that they begin to gain knowledge of what breathing entails. This, of course, can start earlier for some than for others.
Belief, on the other hand, connects knowledge as gained through conscious experience (or education) with conscious thought, word or action. When a child can be said to 'know how to breath' in this sense, they would have some awareness of what it means to 'breath' - for instance, they can respond consciously to an instruction given to 'breath'.
Note: this response, or the knowledge that informs it, need not be correct for it to be a belief. A child in distress, for instance, may simply raise and lower their shoulders or open their mouth - believing these actions to constitute the action 'breath' - without managing to take a breath.