We've done pred once or twice for Alyssa when she was little and having some respiratory issues. We also put my oldest son on it EVERY spring for a "burst" to help his miserably debilatating seasonal allergies.
We've had no issues as the courses have always been short term. Its the long term that is most concerning. But here's some things you should know:
1. It tastes AWFUL!!!!! You really need to get in far and squirt it down her throat quickly or she'll spit it out. We had a horrible time with this one for Alyssa.
2. It can really make them irritable. I did a burst of pred for my asthma once and I felt like a raving lunatic for a few days. It was like the worst PMS ever time 10. My son also becomes very emotional and irrational on the pred. I don't remember Alyssa having this reaction, but it is possible and it is temporary.
If you are concerned, talk to your CF doc. Mine assured me that short term use of a steroid really helps with the inflamation and can help turn around an issue quickly.