We live in modern times; that's for sure. Once upon a time, children and adults spent the majority of their days outdoors, exposed to the sun's natural rays yet protected from sunburn by the ozone layer. Nowadays, we spend most of the time indoors, and when we are outside, we slather our skin with sunscreen to protect against skin cancer (great idea of course).
The downside to this lifestyle is a relative lack of Vitamin D the way nature intended -- from the Sun. Back in the 1940's, when rickets (bone disease seen in children resulting from vitamin D deficiency) was becoming a widespread problem (due to kids spending less time outdoors, more time indoors in the factories, for example), the US government mandated synthetic vitamin D be added to cow's milk. At the time, this made sense; most kids drank at least a glass of milk a day, and the risks of milk drinking were unknown (and probably not as bad as they are today, what with the added hormones, antibiotics, and other stuff that we shouldn't be ingesting). The point is, cow's milk was the perfect delivery mechanism for vitamin D into kids' diets.
Now that we are better educated and things have changed somewhat from the days of Bessie the Cow providing milk for the local families, milk is no longer considered a healthy or necessary food. It also means that many kids are not getting the vitamin D they need.
A recent study out of Boston reports that 40% of the 380 kids examined have lower-than-optimal levels of vitamin D in the blood. Being vegan isn't a risk they identified, but, interestingly, being obese is. Not getting vitamin D in the diet (from fortified food, whether milk, soy milk, cereal, whatever) is, obviously, also a risk. Only 3 of the kids studied had clinical signs of rickets (still very unfortunate), but the others (with low blood levels) are said to be at risk for future bone disease and/or autoimmune problems.
Again. We live in modern times. It's time to accept that some things have changed (in this case, the near destruction of the ozone layer and generous use of sunscreen), and that "unnatural" risks face us. Many vegans claim that a natural, all-plant diet is all we need to meet all of our nutritional needs, but modern living has made this an untrue and dangerous assertion. Back in the day, before modern agriculture, global industrialization and factory farming, we got the vitamin D we needed from the sun, without risk of overexposure. We need to encourage ALL people -- children and adults, veg*ns and omnis alike -- to take a vitamin D supplement and/or regularly seek vitamin-D fortified foods.
Vegans, opt for the D2, not D3, as D3 is usually not vegetarian. Check labels. Fortified soy milks, vegan nutrition bars, and vegan cereals are other possible sources of vitamin D. Again, check labels.