You as a parent have a very difficult decision to make that you should tell your child about the disease, given the fact that there is no treatment of it. You can be honest and straight but be encouraging as well. For example, give them hope that extensive work is being done to find a solution for the problem and it might be possible in the near future.
As parent you can think of the aspect of social stigma that your child can face because of the disease but be assured keeping your child in the dark in the longer run is not of any health benefit for them. So it is better to tell them at an early age rather than leaving the mater, and this choice is particularly not good and can work in negative way for the child if they find on their own about it.
If you cannot find the courage to speak the truth to your child despite all efforts, you can get help of the teacher of the child or one of your relatives. The risk here however is that the child will not feel good about you as parent to have kept it secret from them. One way or the other there is a way to be found to tell the child about the problem. You can also ask the doctor to disclose the news to your child and then use encouraging indicators about finding of a possible solution for it in the near future.
When you tell your child about the problem, be also honest about the actual nature of the disease. For example, you should know and tell your child that the problem is genetic that affects lungs and other parts of the body. It is not respiratory disease that causes difficulty in breathing. The difficulty in breathing is actually caused by CF, because it affects lungs. However, it is not a contagious disease.