From a vegetarian mother's point of view.
Keep the diet as varied and wide ranging as possible. Children are attracted to colour; naturally colourful food is good for us.
Even if they only eat a mouthful of a myriad of different foods, that's fine, and it gives them the micro nutrients.
Seasonality is a good thing, and that's from veg, to saladings to fruits and nuts, seeds and grains.
Children need calories; stodge is not a bad thing. Oats, potatoes, rice, corn, and the like are good food, just don't only feed stodge but don't starve the growing child out of some sense that it is healthy. Look up Harris Lines if you question me on that.
Sugar isn't the enemy but a little can go a long way. Fake sugars are not good things to be feeding anyone.
Mind it takes 2metres of sugar cane to make enough for one glass of coke or other fizzy pop, that's an overkill of sugar.
Salt isn't the enemy either, just be aware of it and encourage the natural taste of food rather than over seasoned.
Processed food is handy, but read the labels. Marmite is just boiled yeast, but that still makes it 'processed', iimmc ? If you're vegan you do know how to read the labels properly
Generations of healthy people have been reared vegetarian (and vegan) don't make food a hassle worrying about it, keep it a pleasure.
In our climate, with our very much 'indoors' lifestyles, Vitamin D is really lacking. If you don't drink milk, you need to get some way of boosting the vitamin D as well as the B vitamins. Nutritional yeast flakes are very good, tasty, and make an easy addition to cooking, they add savouryness without adding salt, and it sorts out the B vitamin issue nicely. Engevita....cheapest source is Amazon.co.uk. They sell the big cans in a six pack, if you like the stuff. It was bought for me, but I think my husband's addicted to it 'cos we're going through it at an amazing rate
I am, honey and a little mature cheese apart, pretty much vegan. There is a standing joke about camping with me, "It's not six a day, it's six at every meal", I don't find that a problem

I didn't find it a problem rearing healthy vegetarian children. Now, I wouldn't worry about rearing healthy vegan children either.
M