Books for children about serious illness

[BillWagglestaff]

Hello,

My three year-old son's best friend (6) is seriously ill and is undergoing chemo for the second time (the first time was before my son was born). Can anyone recommend any good books for explaining serious illness to toddlers? We are generally very straight talking with him, but I am uncertain about how specific or detailed I need to be.

Should probably add - I have found several bookswhich are cancer-specific, but his friend does not have cancer and I do not want to run the risk of my son mistakenly using the word around him, as I think it could compound his family's distress.