There have been many cases of malnourished infants of vegan parents in the news, and cases of parents fighting over feeding their children adequately. It's about demented fanatics who shouldn't be allowed to raise children, not about vegan diet.
How may cases of malnourished infants of non-vegan parents haven't been in the news? Umberto Eco pointed out decades ago how a reporting bias affects public perception.
Why is there a need for this specific law? Why isn't the well-being of a child covered under more general laws already?