Next steps: How do I tell my daughter the cancer is back?

I saw my doctor yesterday and it was a good news/bad news scenario. Good news is that the cancer is growing slowly. Bad news is, of course, the cancer is growing. But, given the slow growth, I have the opportunity to take advantage of a clinical trial that is open now. Chemo is still an option but I don't have to rush into it. What I still have to figure out is how to tell Sydney. She knows about cancer and she doesn't like it. She knows it means I can't have more children and that it takes me away from her because my 'medicine' makes me feel tired. I don't think she understands more than that yet. The really bad news is that I will have to see the sadness in her eyes when I tell her that mommy is sick again.