In discussing the Catiline conspiracy, Dio states "He [Catiline] sacrificed a boy, and after administering the oath over his entrails, ate them in the company of the rest." That's obviously negative propaganda, intended to show Catiline in a monstrous light and, in my mind anyway, equate him with those baby-killing Carthaginians. (Sallust does not mention the event, as far as I know).

Even though it's almost certainly just a story, however, it's interesting that there's no mention of the boy's citizenship. If the prohibition was only against sacrificing Roman citizens, then I have to think that Dio would have specifically noted that that it was a Roman boy. This suggests that, to the Roman mind, it's ok to sacrifice humans in extreme circumstances (e.g. post-Cannae) when it's for a "good cause," but not ok to do so for a bad one (e.g. the Catiline Conspiracy).

The other possibility is that the prohibition morphed from "no Roman citizen sacrifices" to "no sacrifices" between Cannae (3rd century BC) and the time when Dio wrote (late 2nd to early 3rd century AD).

In any case, it's certainly an interesting topic and one that, to my knowledge, our understanding of Roman religion doesn't cover in enough detail for us to truly understand.