It's a very sad accident but a reminder to all that you should trust no dog with children regardless of breed. Same goes for other animals such as cats.
//I think the problem is that you never think that your placid family pet could harm anyone.//
Regret to say I've never come across a placid Jack Russell. On a broader note, when my sone was a new born we made sure that even our small cat was kept away.
We were very careful introducing our spaniel to both the children as newborns. She is a soft lump, but I still keep an eye on them all when they're together.
Tragic, but i agree with McMouse and youngmafbog...........a new "being" in the territory of an animal is always a recipe for "challenge" and the outcome of a contest between a young human and a carnivore is always predictable.