First, let me say that I am not a parent, and that I admire parents of CFers immensely. I can only imagine how tough it must be to watch your kid fight this disease and do therapy every day, but I have to say that as a CFer myself, I'm not sure giving the day off of treatments is the best answer. I have no doubt it makes kids feel better, and I will definitely admit I used to BEG my parents for days off, but I guess my only point is that one of the hardest facts of this disease is knowing that when things get tough we have to rise to the challenge and fight back even harder. CF can take a huge emotional toll on people (patients and caregivers alike) but unfortunately the reality is that a break from actively managing the disease is not the answer. I think this is a lesson best learned at a very young age, because it only gets harder to balance life and treatments as you grow up.
As an alternative, maybe tell your child that you understand how much he/she hates CF, that you feel the same way, but that sometimes in life we have to deal with things that make us sad. Then explain that it's okay to reward yourelf for dealing with tough things and being strong and maybe offer a special trip to the ice cream store or McDonalds, the movies, or whatever your kid loves if he/she does all her therapy that day? It's a treat AND extra calories, all in one. Also, you can include non-CF siblings in the reward, since they have to deal with CF as well. Make it sort of a family reward for dealing with the hard stuff.
I totally don't mean this to put down anyone else's suggestions or ways of dealing with the disease. CF is crap, but I know I feel better when I make myself do my treatments and then treat myself to something extra special for the effort!
