veganism isn't the problem, the problem is malnutrition

These are one in the same for a developing child. That's why they are using veganism as a proxy.

Otherwise healthy adults can survive on a vegan diet. But there is very little evidence to suggest it is actually a healthy lifestyle, but there is a lot of evidence suggesting it lowers blood cell counts and screws-up the balance of cholesterol in the bloodstream... The consumption of animal meat is a crucial part of human evolution; absence of animal products is not just something a person can get used to.

That's why it's being used as a proxy. You could, in theory, feed a child McDonalds and still get him/her the nutrients he/she needs to grow. That is not possible with any known form of a vegan diet. If you're feeding a child a vegan diet, he/she will develop at least one vitamin deficiency. There is just no known way to feed a child enough to correct for this within the confines of a vegan diet.