I have put for you below the link to BCC's publication 'Mummy's lump' a book that has been specially written to help younger children understand what is happening. As with all BCC's publication this one is free, just follow the links to order your copy. I hope you find it helpful.
Here is a link to a thread with more information about the younger women's network as mentioned above by Deb:
Anna, BCC Moderator