There is a difference between vegetarian and vegan. A vegetarian may permit such things as milk, cheese, and even eggs. Such a diet would be OK to raise a child on. However, a vegan does not permit any animal protein at all. While a vegan diet can be healthy, it requires enormous pains in balancing. It is very common for vegans to be anemic, due to lack of iron, or to suffer from lack of vitamin B12, or lack of zinc, or calcium. I would not suggest it is a good thing to raise a child on a vegan diet. There have been cases before where such children have died, due to nutritional deficiency.
The nutritional requirement for vitamins and minerals has only been recognized and quantified in recent times. In the past, people only understood the need for protein and calories.
Homo sapiens is a predatory carnivore. A few million years of evolution was enough to adapt our metabolism from the grazing diet of the other apes to the meat-intensive diet of our ancestors who, first, used flint blades to scavenge the meat off the bones left by the predators, and later, used more sophisticated weapons and tools to become full-time carnivores, and later, used even better technology to become the planet's apex predator who eats both bears and sharks. But a few thousand years of evolution since the Agricultural Revolution has most assuredly
not been enough to readapt our predator's metabolism to a grain-based diet.
For starters, grains are completely inedible unless they are cooked, because the cellulose in which the protein is enclosed cannot be efficiently broken down by acids and enzymes. Animals that eat cellulose "cheat" by having a huge bacterial culture that digests the cellulose for them--and an enormous belly to accommodate the process.
As the Neolithic Era of agricultural villages transitioned into the first Stone Age cities and then the Bronze Age and then the Iron Age, the human population became so large and dense that there wasn't enough room to dedicate land to growing food to feed to livestock--an extremely resource-inefficient way of life. So people began subsisting on grain, and only the rich and powerful could eat meat on a regular basis.
As a result, in the Roman Era, the life expectancy of an adult who had successfully survived the ravages of childhood was only about 25 years. This contrasts with the meat-eating hunter-gatherers of the Paleolithic Era, whose surviving adults had a life expectancy of around 50.
It becomes much easier to fulfill the human nutritional requirements by adding modest quantities of eggs and/or milk products to the daily diet. Raising livestock for dairy is a tenfold increase in resource efficiency over raising them for their meat. Not just cows but goats, sheep, yaks, even horses and camels. But not llamas, they barely produce enough milk to feed their babies and the poor Incas had to develop a civilization without milk. If you know anyone who's tried to milk an elephant, please tell us how that worked out.
Raising chickens/geese/ducks/whatever for eggs is also more resource-efficient than raising them for their meat.
To subsist on an ovo-lacto-vegetarian diet (animal tissue is allowed so long as it does not require killing the animal) is relatively easy, because milk and eggs contain the vitamins and minerals we need. To subsist on a vegan diet (no animal tissue at all) is very difficult. Tens of millions of Roman citizens died in their twenties, trying to make that experiment work.
Let's learn from their deaths, so they won't have died in vain.
Note: In the twelve thousand years since the Agricultural Revolution, dogs, with their six-month breeding cycle, have gone through about twenty thousand generations compared to a few hundred for humans. They have had enough time to adapt from hunters to scavengers. Their brains are smaller than wolf brains so they don't need as much protein in their diet as their ancestors.
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