How can I die within 6 days

Lymphoma can kill in 2 weeks. This is cancer of the lymphatic system. Can I suggest that this would be a good one to choose for a book, as symptoms get worse and worse. It is also one of the few cancers common in children - you said that the sufferer is a girl. It would make for a truly upsetting story, yet possibly uplifting story, with good times along the way.

If you need cancers common in children, here are the few others

Brain

Another is cancer of a limb, this is usually fatal, as they metastasise (spread) very quickly

Here are general survival rates for different forms of cancer.

The length of time people have when a cancer becomes terminal is determined greatly by the form of cancer. Lung cancer sufferers usually live between 6 months - 1 year, whilst brain cancer has so many forms, it cannot be averaged very well - some forms allow people up to 10 years, while others can be only a few months. Generally childhood brain cancers are at the more agressive end of the spectrum.