I used to be in PETA but I left. Many vegans treat the diet as religion which is dangerous. It becomes fanatical objects of worship. I found a nice christian,vegetarian group that isn't so fanatical. Having said that, I deal with vegans every day and only had one customer have problems with diet.
As Ivan said above, it has to be a conscious decision. A three year old kid doesn't get to make that choice.
Where do you work, cyborg?
Adults can do okay on a vegan diet for quite a while, but not indefinitely. But I've never heard of a child who'd been on a strictly vegan diet since birth growing up without serious health problems.