The problem I have with this is, babies die all the time, there's no way to know if it was a particular diet or raising environment that caused it, or if the parents were just unlucky. To punish parents based on such a development is result-oriented, even if I personally believe that a vegan diet is quite unhealthy. Lots of other things are unhealthy too, but they don't put people in jail for that, nor should they.
If this had been a couple of poor, black drug addicts, instead of being upper middle class whites, and fed the baby an even less nutritious vegan diet of only sugar, vegetable oil and wheat flour-based products, they wouldn't have faced charges for this.
Clearly, something can't be a crime or not based on how much money you have, what race you belong to, or what belief system you hold.
What's more, if the State gets to do this in this case, then they can do the same or even worse in any number of cases where they determine, at their sole discretion, what sort of diet or lifestyle is the one you should use to raise your kids. At that point then your kids are no longer yours, they belong to the State. That is a very dangerous situation. Imagine that some vegan activists grab a hold of the State apparatus after that, what will happen to everyone else who believes vegan diets are not good for their kids?