It makes me wonder. It says
Though her parents didn't want her to know the severity of her cancer, they feel that she must have known what was happening.
It's making me wonder what was going through her mind:
1. I had/have a disease.
2. They've stopped treatment, won't tell me I'm cured (and I know I'm getting worse), and won't tell me when I'll stop getting worse.
3. They are very sad.
Looking through kid lenses, she might be thinking, "They are sad because they aren't able to fix me. I should make sure they know it's ok that I can't be fixed."
It's the thoughts she must have had that compelled her to buoy her parents up - the idea she had that she was responsible for making sure they weren't sad about her illness - that makes me sad.
It's not a burden a child should bear.
Not that this is in any way relevant to this particular child and story, but as sad as the story itself is, it's more a celebration of life, no matter how short, than a tale of woe.
-Adam