Vegan children are unhealthy. Numerous studies have demonstrated this. The youtube channel "What I've Learned" as well as Jon Venus show in total 9 studies.
The position of the world's largest organization of food and nutrition professionals is that "Vegetarian children and teens are at lower risk than their nonvegetarian peers for overweight and obesity. Children and adolescents with BMI values in the normal range are more likely to also be within the normal range as adults, resulting in significant disease risk reduction. Other benefits of a vegetarian diet in childhood and adolescence include greater consumption of fruits and vegetables, fewer sweets and salty snacks, and lower intakes of total and saturated fat. Consuming balanced vegetarian diets early in life can establish healthful lifelong habits."
A number of comprehensive studies are linked within that article.
Using these papers to counter the scientific consensus on vegetarian and vegan diets has been conclusively debunked.
In short, these papers are typically small in sample size, have improper sampling, or have other issues such as looking at vegan children who are not supplemented at all for even basic vitamins like B12 and also not controlling for vitamin deficiencies in sampling. In addition, some of the shorter children have shorter parents, and at least two studies were from over 30 years ago (when significantly different vegan foods were available) so there are just a lot of issues that such simplistic analyses have. In addition, many of the anti-vegan analyses of these papers cherry-pick the unfavorable characteristics of some vegan children and omit all the health benefits that these children have, such as better LDL and BMI levels, as well as better eating habits in general. Hence, they do not do much to counter the scientific consensus and these organizations recognize that.