I’m sure you’ll have no difficulty finding great recipe ideas, but maybe start by asking the girl what sort of thing she likes? At her age she may well not yet have acquired a taste for spicy food.
I’ve been vegetarian myself for well over 20 years and could easily “beef” on about it, but for now I’ll just stay on topic... When my three granddaughters came for their usual two summer stays last year, the middle one, then aged 9, had recently turned veggie, and some of the things I found she especially liked were crispy jacket potatoes, fresh babycorn, raw peas, cherry tomatoes, plain pasta, and boiled or “sunny-side up” eggs, usually with toast. She also loved M&S stuffed red peppers, and the lemon (now lemon & herb) variety of Tilda microwavable basmati rice. Plus, as usual, all three girls ate truly unbelievable amounts of fresh soft fruit, which apparently isn’t available where they live on the other side of the world. Finally, it turned out that (unlike me) my so-called vegetarian granddaughter did still eat fish, which meant she didn’t miss out on the girls’ all-time favourite of fish fingers and chips!