This cartoon book seems to be useful to explain skin cancer to kids.
http://www.heraldsun.com.au/news/national/cartoon-book-by-medikidz-helps-parents-explain-deadly-melanoma-skin-cancer-to-young-children/story-fnii5v70-1226675589798

or get more information on melanoma here:
http://www.melanomapatients.org.au/

regards
Ian