Belgian officials will "no longer tolerate" parents who force their kids to be vegan.
It follows a number of cases in the European country of children being hospitalised and suffering health complications, and even a death in 2017, linked to vegan diets.
Government children's rights official Bernard Devos asked the Royal Academy of Medicine of Belgium to make a ruling on the controversial diet, whose adherents avoid eating meat and dairy, and avoid using products made from animal-derived sources.
The academy did so on Thursday, reports local paper Le Soir, saying it was "not ethical" to impose veganism on children.
"This restrictive regime requires ongoing monitoring of children to avoid deficiencies and often irreversible growth delays," the academy's report said.
"It is unsuitable for unborn children, children, teenagers and pregnant and lactating women... This concept of nutrition is similar to a form of treatment that it is not ethical to impose on children."
It's reported around 3 percent of Belgian kids live in vegan families.
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Prof George Casimir of Queen Fabiola Children's University Hospital, who led the team that put the report together, listed the potential side-effects to UK paper The Telegraph.
"We are talking here about stunted growth and psychomotor delays, undernutrition, significant anaemia. Some developments must be done at a specific time in life and if they are not done, it is irreversible."
Devos said the legal opinion from the academy will make it easier for children to be removed from parents enforcing a strict vegan diet, and charge them with a crime.
"We can no longer tolerate this endangerment," said Casimir, saying parents now face up to two years in prison.
A Belgian vegan couple convicted in 2017 after their seven-month-old died of malnutrition only received a six-month suspended sentence. A Sydney couple are now facing five years behind bars after their toddler suffered a brain seizure linked to her vegan diet.
Animal rights group PETA angrily rejected the ruling, calling it "ignorant codswallop".
The American Dietetic Association, as PETA pointed out, says it's fine for children to go vegan, even from infancy - as long as the diet is "well-planned".
The New Zealand Ministry of Health doesn't rule out vegetarian or vegan diets for children and has plenty of advice, but does warn their "stomachs may be too small to hold all the food they need for activity and growth". It recommends parents speak to a doctor or nurse for advice on what to do.