I have read of rats attacking a baby in the past. I think the odor of formula or whatever attracts them. This is horrible. Unspeakable. There had to be more than one rat, and the parents had to have ignored the baby's crying, if in fact that was the cause of the death.
Or maybe the rat suffocated the baby first because it smelled the milk. I have heard of cats doing this. I think sewer rats are like gopher rats.