We've had a few discussions about this. Here are the links, hopefully they will help:
http://www.whatnext.com/questions/how-did-you-tell-your-kids-you-had-cancer
http://www.whatnext.com/questions/tips-for-talking-to-your-kids-about-cancer
http://www.whatnext.com/questions/tips-for-telling-your-parents-you-have-cancer