Don't worry too much, I am a living proof that a child don't need to eat to survive.
Until I was 30, I never eat that much.
In primary school, my mother told me that the responsible of the place we went to eat at lunch called her and said:" You don't need to pay for his lunch, remove him from here, just buy him a baguette in the morning, he is only eating bread and drink water anyway!". Bread is good in France, and I know, I eat a lot when I was kid.
As long as she is in the norm (height vs weight), that is OK.
I was skinny all my youth (doctor told my mom, I am not too skinny, I was powerful (only muscle, no fat)), so skinny that when my wife saw my pictures, she told me she would never marry me at that age (mid twenties) as I look like a sero-positive sick person.
Thankfully, I took some fat before meeting her, and now, I am even fatter thanks to her food.
When I was young, I never wanted to eat any vegetables and fruits. Only meat (a bit), yogurts, cheese and milk (quite), bread (a lot), and candies (way too much).
Now, I am 6'1 and 190 lbs,... with a belly.
So don't worry too much, she knows what she needs, just be sure to provide lots of variety and she will pick what her body asks her. Don't force any food on her, just show her how much you like it, and she may starts to develop her taste.