Infestation is a far cry from "A Rat."
A mouse can enter your home through a hole that a pencil would fit through, and a rat only requires a hole about 1 1/2- 2 inches to gain entry to your domicile.
If they really believe that it was the pup, they should have purged the pups stomach to check the contents and should be screening the pups feces for proof. Unfortunately, this probably did not/will not happen. In another article about the same, they mention that they also own a ferret, but that it was in a cage and impossible for it to get out.
I keep a clean home yet still have a problem with mice. Why? Because it is being renovated and it will take time to fix/close all the entry points. It is certainly not an off the wall thought that a rat could have done this, living conditions aside.
No matter what animal (or human) did this to the infant, it is the parents responsibility to keep that child safe. Which they have failed miserably at. I just hope that they don't kill this pup due to human failings.