It is extremely normal for rats to cannibalize their dead. And the baby would pretty much have to be dead before being consumed or the cage would have had fresh blood all over it. Even a tiny baby will struggle while being killed.
A rotting corpse in your house draws predators, it's their safety mechanism.
We are as gods to the beasts of the fields. We order the time o' their birth and the time o' their death. Between times, we ha' a duty. - Terry Pratchett.
"Men have forgotten this truth", said the fox, "But you must not forget it. You become responsible, forever, for what you have tamed. - Antoine de Saint-Exupéry