One thing that's often overlooked is that a vegan diet is simply not natural for the human body. We cannot survive on a purely vegan diet. Period. There are no pure vegan cultures ever.
How do modern vegans survive, then? They get the vitamins they need from some fortified source, or they supplement with vitamins. That's fine - I have nothing against them choosing to do so, it's certainly their right to pursue that lifestyle, and there's nothing wrong with it. They should be careful and conscientious enough to make sure they get the nutrients they need, but that's their lookout, just like it's my lookout as an omnivore not to kill myself with junk food and fast food hamburgers. However, to turn a blind eye to the nutritional needs of a child based upon your own dietary choices is negligent. Criminally so.
Note that the above applies to vegan diets, not vegetarianism - it's possible to have a healthy vegetarian diet once you supplement with things like eggs, fish, and/or milk.
Good article on the subject:
http://www.nytimes.com/2007/05/21/op...0A&oref=slogin