I was vegan for a few years. I gained 25kgs and had no energy and it was just shit.
After that I went high fat no carb, and lost almost everything back and cannot be happier with my health and energy.
In hindsight I was young and dumb and didn't take the meal prep seriously. I think if I did it again, it would be better because I wouldn't just cook pasta every night with tomato sauce.
I did it for ethical reasons, but **** it was hard. I'd be really worried about focussing at school for a 14 year old. I guess if you have the resources to spend a lot on food, and make sure she gets everything she needs it could work.
My best friend owns a lot of vegan restaurants in Sydney - if you are in Sydney you can PM me and i'm sure my friend would love to have a chat to you/your daughter.
I think the only thing you can't get in a Vegan diet without some form of supplements is B12. Best to talk to a doctor and make sure she's not going to miss out on anything in her development years