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.
You say "much more", but the models of healthy eating don't really differ that much in practice. Both prescribe a balanced plate with carbs, proteins, and "other" vegetables. The omnivore model calls for half the plate being vegetables, while the vegan plate is divided into equal thirds.
That of course being the rule of thumb, and obviously people don't necessarily build every meal to exactly that pattern. A pizza night every now and then is okay too.
> rarely have any significant nutrient deficiencies
People in the Nordics are usually already supplementing at least vitamin D due to lack of sunlight, and often iodine due to a lack of it in the soil. So in this context, switching to a combined vegan supplement that includes those as well as B12 is a relatively small change.
