That very same link states that "a carefully composed vegan diet" can be beneficial. Thats my point -- with a vegan diet you have to be much more conscientious about nutrition. Many people are not. With your average healthy omnivorous diet, you rarely have any significant nutrient deficiencies that you have to worry about. Said diet ends up being simpler to implement in practice. That's what I mean by careful management; A healthy vegan diet for children is not impossible, but it takes more effort.