I sit on the fence on this one. My mother died when I was 12. When I was 9-10 she sat me down, told me that she was sick...told me that she did not know how long she had. She spent the next 3 years teaching my father how to cook, teaching us kids how to do laundry and help take care of the house all on top of being den leader, brownie leader and soccer mom. When the end was close, no one said a thing to me...just came home from school and dad told me she was gone. I still remember to this day that I wished I had been able to give her a kiss goodbye. For that reason, I would want to tell my children at least. The other side of the fence that I lean to is that I know that I am not nearly as strong as my mother was so, realistically, I don't know that I actually COULD if the situation were to arise. Ever since mom died, the family has never been up front with me about illness...I just get a phone call when it's too late to say goodbye and that really chaps me. I guess I am a hypocrite...I want people to tell me but I don't want to have to tell anyone else. Hard question to answer.
Ask yourself, "When Iím on my deathbed, will I be glad I was so serious about _______?" Let down your hair and stop being so darn important for a while.