This seems like a "definition of right" quote rather than a moral statement. I'd rather just say "being certain that poison is good for your child makes you subjectively right, but not objectively right, to administer it." Or if those terms are already being used for something else, we can make up new words.

Then of course we might ask, for example: when determining if criminal action is appropriate, does it matter whether the criminal had a subjective but not an objective right to commit the crime? And that would be an interesting question. In absence of a context, it's pointless to discuss which of two things should be called "right".