I would suggest a cool, clean area – indoors. Just do fun low level obstacle type activity. Lots of hydration. Keep the groups small, no more then 4 kids so they can be watched for signs of distress. I have a son with persistent asthma, I know CF is different but pulmonary exercises MUST be done indoors in a climate controlled clean area.
there are camps for children with CF, and I am sure that the people supervising those camps would be able to provide you with ideas and tips.