What do you mean, "voluntary"? Infants and children are subjected to all kinds of things in the course of their upbringing with zero consent of any kind, diet included.
And I'd love to see any sources you might have for saying that going vegan from birth is "not proven" (ignoring the fact that nothing is "proven" in science). This recent (2019), multi-center, peer-reviewed article establishes that plant-based diets are perfectly feasible from birth, but I'm curious to see research from the "other side": https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6356233/
IMHO,
Vegetarian, probably OK, but should be voluntary, and make sure to get enough animal protein (egg, cheese, etc).
Vegan, not proven.