From the article I linked to above, their child died because the parents fed it too little food. Simply feeding the child more of the same foods would still have damaged the child, but not fatally.
The parents seem to have been found responsible for the child's death because they recieved adequate warning about what they were doing to their child but still did not act to protect it. The soy milk cartons carried clear warnings and the child's grandparent also warned the parents.
In Australia, it may not even be necessary to prove that the parents read the carton, simply that they were able to read. The relevant case in Australia is
Balmain Ferries 1905. I suspect it is the same in other countries with British-based legal systems.
It is clear that this case is not about being ignorant about what a child needs. It is about deliberately failing to take care of a child while knowing how to properly take care of a child.