This might be more of a thing with daughters: it seems at a certain age, if they love animals they don't want to eat them. And fair enough. It's good for them to think about where their food comes from, what is wasted, and how we care for animals.
My high school friends, my daughter, friend's daughters, my niece, and my cousin went through this at various points. While I don't want to destroy youthful idealism, it calls for a conversation about nutrition.
One high school friend's dad was a doctor and made her eat chicken and eggs until she got into her teens.
My friend's daughter wanted to be a cookies-and-crackers kind of vegan, so my friend explained to her that this wasn't sustainable, and she would need to take vitamins, and eat a good variety of real foods to be healthy.
For my own daughter, I explained that while vegan eating has a long history as a spiritual practice for adults, there is no native culture that chooses to forego all animal foods, because of their convenience and nutrient density. Getting the same nutrients from plants requires more planning, more chewing, and non-negotiable supplements.
It's challenging for active kids to take enough time to eat enough calories if they all contain fiber. This could be great for a sedentary or overweight kid, but nutrient deficiencies would still be a consideration in the long-term.
So what happened?
My sister let her daughter roll with vegan food prep, and my niece filled her plates with colorful stir-fries. After a few weeks she asked for meat again. Despite the enormous volume of food, she said she never felt full. My sister had been expecting this because her daughter is lean and sporty.
My daughter found that she wanted meat after a few weeks, and was sick of the other protein sources I was offering.
My cousin persisted for several years until life-threatening anemia caused her to re-examine her eating habits. My friend's daughter decided she wanted to have more variety (aka junk) and not be restricted to real food.
One high school friend is still vegan. One became an omnivore.
Some people truly do feel their best being vegan all the time, and their bloodwork backs this up. The majority of women bearing children seem to feel best as omnivores, possibly because it works well with our busy pace.
Many adults, particularly overweight ones, seem to benefit from periods of vegan eating. Most religious traditions have seasons without animal foods, or with fewer options, and I suspect this would meet our natural need for periods without meat and the general excess we fall into. But children were typically exempted from the full rigors of fasting and dietary restriction, on the understanding that this is a time of intense growth. Once the growth years are over, we'll have the conversation again.