Telling a 10 year old about thyroid cancer

I was diagnosed on 10-9-08 with papillary thyroid cancer. I live with my sister and her 2 children, I practically helped raise them. I have told my 15 year old neice knowing that she would understand it better coming from me but I have not told my 10 year old nephew, I thought it would be better coming from his mother. She was in China when I found out so my nephew had already figured something is goind on but does not know what. I asked her if she had told him yet and she said no because he just would not understand. I do not feel the same way, I feel that he should be told in a way that he would understand. I have only had partial thyroidectomy and I am scheduled to have the rest taken out on Tues. as long as I have my voice back from the first operation. after that I wil be starting the RAI in a few weeks and if by then he is not told he will be very confused when his aunt is not her self and is moody,etc. If anyone has any suggestions on how to make my sister understand he need to be told and any ideas on how to tell him at a level that he would understand I would greatly appreciate it. Thanks.
Cindy