For a vegetarian/vegan diet, plant-based protein and vitamins are essential and should include plenty of legumes, beans, nuts, complex grains, organic soy, and leafy greens. I recommend checking in with a registered dietitian to ensure your child is getting an adequate iron and calcium intake, which if found to be on the lower side can be safely supplemented with the right choices.
For children omitting fish from their diet, you’ll want to be sure they are getting enough DHA (Docosahexaenoic acid)—a healthy fat source for brain development—from other sources such as certain nuts and seeds. Ground flax seeds pack a good punch of DHA and can be pretty easy to incorporate into the diet, but for some kids an added supplement might be the right choice to ensure adequate DHA intake.