Is the vegan diet harmful to the child? Probably not, but you'll have your child taken away if your child is malnourished.
But not getting them vaccinated doesn't harm their children...
...provided that everyone else around their children is vaccinated.
But when that population drops low enough...then herd immunity stops working. Then their children get sick, and other children may also get sick, etc.
So a parent not harming their children is increasing the risk to other parent's children that they will be harmed. Should parents be allowed to stochastically harm other parent's children?
Nobody can prove it in the individual case - but statistically it can be shown to cause such harm...
None of that brings into the discussion that such unvaccinated children could become sick and pass it on to other immunocompromised individuals (adults and children)...