To turn the tables:
Superhappies just don't like suffering and can't understand how it could be beneficial to any species, because it hasn't been beneficial to them (or the detriments have been too great to offset benefits).
From that standpoint, removing all suffering would be a valid point of view but they are failing to take into account the psychological and physiological differences between Humans and Superhappies, which are significant enough that Superhappies shouldn't be making assumptions that what is good for them is good for everyone, and what is bad for them is equally bad for everyone.
This betrays a fundamental immaturity in their reasoning: The thought of pain is so uncomfortable to them that they are able to justify its removal from any species they encounter, by any means necessary.
To be honest, I can't believe a species like that would fall into this sort of reasoning in reality, but I guess for the story's purposes it had to happen...
I'm of the opinion that a negative act is pretty much morally equivalent to a positive one.
You mean leaving good thing undone is equivalent to doing a bad thing?
I could agree to this if it wasn't for the context; there is no guarantee whatsoever that removing the ability to suffer from children would be a good thing - thus leaving it undone can't be treated as equivalent to doing a bad thing to a child.
If it turned out that people who have the ability to suffer will do better in life than those without that ability, that would mean the children without the ability would grow up to be emotionally (and possibly physically) crippled adults.
That would be a
bad thing.
If the humans think eating babies is not a necessary component of normal (and healthy) babyeater existence, then they have the burden of proof considering THEY are the ones suggesting the change, or mind their own business.
The baby eater children don't have a healthy and fulfilling life, as opposed to children who grow up to become able to cope with occasional bouts of unease...
The switcharound analogy is incomplete because there's no conclusive proof that leaving children with ability to suffer counts as "leaving a good thing undone", while eating a baby eater child is definitely a "bad thing to do" for the baby eater child (considering it brutally ends their existence in a prolonged death).
The argument thus relies on whether or not removing the ability to suffer would
really advance and improve the state of human existence. Until that question was resolved with some degree of certainty, the Superhappies have no justification for their claims, and they are undressed as what they are - an attempt to force THEIR standards on OTHERS, just like adult baby eaters enforce their standards on the baby eater children by eating them.
TL;DR:
Eating of the baby eater children is only for the benefit of adult baby eaters, while there is a definite possibility that letting human kids retain their ability to suffer could actually result in net benefit for them, and it would require quite a long research period with a test population to ascertain the functionality of humans without ability to suffer.
*breathe in*
Ethical implications of required tests, unpleasant.