Be proud - I told my kids about cancer :)
I used a walk for the American Cancer Society that we are doing this friday as a topic starter and the car ride as the setting and I think it went well. After my short talk was done, I asked her what she got from what I just said to her, and she told me "purple balloons". Well, this response was reminiscent of the Mary conversation but I was sure that something else had to have sunk in. So after prompting "anything else?" I got these series of answers:
"to help people feel better"
And when I asked her for what disease we were doing that all for, she did blank out - kind of like the Freudian blank out of adults, but quite unintentional and consistent with any new word thrown at a 5 year old.
But hey, purple balloons to raise money to make people feel better...I think that covers it simply well!