That is a very interesting and important question.

To begin a response, let us share that what is in “the best interest of a child” is a broad qualification and likely includes a multitude of important considerations.

It is beyond my ability to speak on behalf of another family’s responsibility to decide what is in the best interest of their child. That being said, some points of consideration may include:

• Is it healthy for a 6-year-old to eat a vegan diet? Dr. Michael Greger at NutritionFacts.org has created a large library of content addressing the impact of each a plant-based and an animal based diet on the health of children. To access the material, you may click here.
  
  People for the Ethical Treatment of Animals (PETA): The animal rights organization also published a comprehensive article titled “Vegan Children: Happy and Healthy” on the same topic which is accessible here. Their conclusions include:
  
  “Children raised as vegans—who consume no animal products, including meat, eggs, and dairy products—can easily obtain all the nutrients essential for optimum health from plant-based sources.”
  
  They also quote Dr. Benjamin Spock as saying:
  
  “Dr. Benjamin Spock wrote in his book Dr. Spock’s Baby and Child Care, “Children who grow up getting their nutrition from plant foods rather than meats … are less likely to develop weight problems, diabetes, high blood pressure, and some forms of cancer.”

• Is it morally or ethically good for a 6 year old to be vegan? That question is, of course, best answered by the child’s family. My opinion is that there is much to be gained by expanding the reach of a child’s compassion and empathy to include all the animals with whom they share the earth.
  
  It is likely that we would raise a child to care for, treat well, and not consume a cat or dog - it seems that only good could come from extending those lessons and attitudes to all other animals.
  
  
• Will the future earth that a 6 year old will eventually inherent be improved by the child living a vegan lifestyle? Very difficult to say definitively as there are arguments for each the positive and negative environmental impacts of veganism. My conclusion based on the research and opinions I have encountered is that yes, the world’s environment and general livability are improved by the choice to be vegan.
  
  Below are two reasonably presented arguments regarding the potential environmental impact of plant-based eating:
  
  Environmental Impact of Vegans Versus Meat-Eaters: The Results of This Study Are Eye-Opening! - One Green Planet
  
  Vegetarian And 'Healthy' Diets May Actually Be Worse For The Environment, Study Finds
  
  The above list of potential consideration is not exhaustive and does not address the important topic of the possible family dynamic impact of a child being raised vegan.
  
  Veganism is not a cure for all the world’s challenges, nor a magic elixir that cures all ills. It is, in my opinion, and my experience, a way of living that aligns well with a personal commitment to live as healthily, joyfully and kindly as I can - perhaps discussing that way of living with a 6 year old would arise with some interesting conclusion from the child about what is in their best interest?