This borders on solipsism. The same argument can be made about any human being. At the very least, we presume there is pain by observing the behavior of animals

This borders on solipsism. The same argument can be made about any human being. At the very least, we presume there is pain by observing the behavior of animals in response to stimuli and interpreting what we see analogically. An in light of certain reasonable assumptions (like "natura nihil frustra facit"), we can quickly rule out a host of preposterous interpretations to conclude that the sensation felt is painful. That We don't need to factor into the moral calculus the possibility that no pain is felt since the most reasonable and rational conclusion is that infants do feel pain. What reason could there possibly be to believe that they don't that isn't some stretched exersize in special pleading and evasion? The baby is human!