I don't think their unvaccinated status should play into whether charges are laid. I would argue if, as soon as symptoms appeared, the parents sought appropriate medical care, that's not manslaughter. On the other hand, if the parents delayed care, that's similar to other child-neglect/manslaughter cases we've seen, and they could be charged for their actions.
I don't like that we have anti-vax people in our society, but I support their right to choose. Their choices do come with some consequences (limits to job prospects, places they can go, etc.) but if we say not vaccinating your child alone is enough to trigger charges, that's very close to saying that choice is illegal.