You can remove the argument from morality safely from your answer just by stating the dog acted upon instinct, based off the notion dogs are pack animals, that have a closely knit symbiotic relatioship with human, which can be used to in favour of the dog finding a newborn activated the instinct of preserving their pack.

The way you approached the subject can be easily side tracked through arguing you are atributting self interest to the animals actions, as in, it keeps the newborn alive, thus, their own preservation is assured.

If acting on true self interest, the dog should have allowed the newborn to die.

Side note: who discards a newborn in such calous way? How unbalanced is the person?