My 3 dcs and 5 of my nephews and nieces are veggies, all glowing with health and energy.
Some of them are fussier than I/their parents would like. Many of them are not keen on the sort of veggie food I like - spicy, lots of tofu, lentils, pulses, veg, lots of foreign influences, especially Indian veggie food. My dc and their cousins don't go for this much at all.
What we do with my fussiest dd is look at sources of protein and make sure she has a mix of dairy/non dairy. also that it's not all quorn (all my dc love quorn and will happily eat quorn spag bog, quorn sausages etc a lot).
Top tips include giving them the food separately, in contrast to a lot of adult veggie food. So, 2 of mine will happily eat chickpeas straight from the tin, or fried with spices. all 3 like hummous with raw veg to dip in. All 3 like eggs so we have a lot of those. Boiled eggs and toast, omelettes, scrambled egg, all popular and easy.
Nuts are also great for veggie kids, again mine have these as snacks rather than in mixed cooked meals, though they will all eat nut roast they don't rave about it, but they like little pots of cashews, or pecans, etc.