I'll preface this by saying something some people may think is controversial:
There are no nanny dogs!
"The trouble is you have no way of knowing do you. This dog, it seems, was owned by a respectable, decent person and hasn't harmed before. When it did, it did so to a horrific extent. How could you ever know whether this dog was to be the one in a million?"
You are right. You never know. No matter the breed. In some cases the likelihood is a bit higher, but the possibility always exist.
you have no way of knowing with any dog. I love dogs, my family used to breed dogs. But dogs sometimes react in a way an owner wouldn't have thought the doggie to be capable of... dogs are animals, they may not think: "oh, cute baby.". They may think something like: "stinky intruder disturbing the hierarchy. And oh my, aren't these screams exciting?!"
Children are smaller than dogs and their screams may trigger hunting instincts, screaming may excite a dog... (and yes, some dog owners may now say: "but not my darling fluffy...". Well, you never know.)
First of all, if you do have a dog and bring home your baby... that's not when you should start worrying about whether that's safe. It's way to late by then, you should have worried months ago. You'll have to "baby-train" your dog. Installing doors to make sure the dog can't enter the nursery or the play room is also a good idea.
Not to mention that a baby may disturb the pack hierarchy, something a dog may be very very opposed to. So, you should to train your dog and more importantly watch him when he's around a baby or a child...
No dog, not even a chihuahua, should be left alone with a child. Unless you 100% sure that the dog can't reach the child (or the child the dog).