Knew of a couple who were like this and they had a child who looked absolutely skeletal. Probably never had a proper meal.
And therein lies the problem with these idiots. The human body, especially infants, needs animal protein to develop and grow. I was in a pub in Hampshire about 25 years ago and this family came in, small Bill Oddie lookalike man, earthen hippy wife and two little daughters. The parents didn't exactly look the picture of health but the kids looked like they'd just been released from Auschwitz. At the bar, the father asked for the vegetarian menu. Assuming those kids are still alive, I would imagine they're under-developed in all areas.
If you want to be a vegan or vegetablist, I'm ok with that, but it should not be forced on others, even their own kids. I girl I sometimes walk my dog with is a vegan and she has three kids (16, 13 and 11); one is normal, one is a vegetablist and the other is a vegan, all by their own choices.
I sometimes have to go vegan when eating in places of very limited choice, such as planes, and I don't know how people can do it because it is not in the least bit enjoyable. I never travel without a bag of protein and a slack handful of protein bars, just in case. I'm fed up of trolley dollies telling me I could've pre-ordered my in flight meal on line; bit difficult when jumping on a plane with only a few hours notice.