Okay. I'm going to defend veganism. I don't see anything wrong with it, as long as you're getting enough calories and nutrients. There are plenty of non-meat sources of protein and non-dairy sources of calcium. I would suspect that these parents are ignorant vegans. Probably didn't do any homework on how to live a healthy vegan lifestyle.
That being said, there is no excuse for any parents, no matter what sort of diet they follow, to allow their children to become malnurished.
There's nothing wrong with any approach to food, as long as it means YOUR nutrient (including fiber) needs, and doesn't cause you health problems.