I've been vegan for about 3.5 years now. My 12 year old daughter made the decision on her own to go vegan as well about 1.5 years ago (she was mostly vegetarian a year before that too). My son and hubby are omnivores, so meal prep can be challenging at our house, lol.
I do it for mostly health reasons, while for my daughter it is more about animal ethics. I've done my research on making sure we get a balanced, healthy diet and her pediatrician monitors her through blood tests to make sure she's not deficient in anything, which she has never been. I don't necessarily think the vegan diet is superior healthwise to other types of diets that emphasize whole foods, but it works for us.