I am sorry to read of your daughter's diagnosis, and can understand why you'd like your granddaughter to be aware of what is going on.
I found this booklet that Breast Cancer Care produced called Mummy's Lump which you can download from here. It tells a story of breast cancer through simple drawings that are good for children to understand.
We have information about speaking with children here, and Macmillan have information about talking to children and teenagers here.
If you or your daughter would like to chat to one of our breast care nurses, please remember you can always do this by calling us on freephone 0808 800 6000 (Mon-Fri 9am-4pm; Sat 9am-1pm).
My 29 year old daughter has been diagnosed with cancer in her breast and armpit. We are struggling with how to tell my granddaughter who is 9 but more like 19. She's a very grown up child , very clever and will be able to spot a " cover up" a mile off