Veganism is dangerous for children
The major dietetic associations state that well-planned vegan diets are appropriate at every life stage, including infancy. Planning matters more at this age than any other.
Children are not small adults: they need dense energy and reliable B12, iron, iodine, calcium and vitamin D during years when deficiencies can cause lasting harm to growth and brain development, and they cannot choose their own diet or notice when something is going wrong. Animal foods supply these nutrients easily and with little room for error, while a vegan diet depends on parents getting supplements and food combinations right every day, which is why some national health bodies advise against it for the youngest children. The cases of seriously malnourished vegan infants show what happens when that planning falls short, and a child should not bear that risk for a parent's ethical convictions.
The Academy of Nutrition and Dietetics, Dietitians of Canada and the British Dietetic Association all hold that appropriately planned vegetarian and vegan diets are suitable throughout the life cycle, including pregnancy, lactation, infancy and childhood. Some national bodies, notably in Germany and Belgium, take a more cautious line for the youngest children, not because plant foods are inherently inadequate, but because the margin for error narrows when energy requirements are high and stomachs are small.
The honest position is that a vegan diet for a child requires deliberate attention to B12, vitamin D, iodine, iron, calcium and energy density. The cases that reach the news involve severe restriction and neglect, not planned vegan diets, and similar cases occur on omnivorous diets without the diet being blamed.
Drawing a general conclusion from a sample too small or too unrepresentative to support it.
How it applies here: Generalises from a small number of extreme neglect cases to vegan diets as a category. Those cases involved severe caloric and nutritional restriction, which is a different thing from the diet being discussed.
Substituting an emotional response for a reason. Emotions are often relevant to ethical questions; the fallacy lies in using feeling in place of an argument rather than alongside one.
How it applies here: The force of the objection comes from the image of a harmed child rather than from evidence about children actually raised on planned vegan diets, which is the question at issue.