I've been seen at four different CF centers (two as a regular patient), and they all say exercise is one of the most important things we can do. We of course have to take our medications and still do airway clearance, but exercise is pretty much on par with those. I believe that advice is valid for both kids and adults.
I didn't get diagnosed with CF until I was an adult, though I've always struggled with CF symptoms - I just didn't know it was CF. Had I known, I'm sure my parents would have focused more on my nutrition, water intake, electrolyte replacement, etc. when I was playing my various sports. Does your CF center have a nutritionist? He/she should be able to help you figure out proper nutrition to support being active. At this point, I know how much I need to eat as well as how much salt I need to replace when I exercise, but it was more trial and error than someone telling me. Looking back, I probably should have asked my nutritionist to give me some guidance.
I truly believe that I've helped counteract my CF being so active. As a kid, I played organized soccer and basketball, and also played outside all the time, so I ran around a lot. I also rode my bike everywhere. In addition, I played the trumpet from elementary school to college, which is a great lung workout. I also played intramural sports in high school and college, and then in adult sports leagues after college. Even golfing is great, especially if you walk - carrying a bag of clubs for 18 holes is a good workout! In my opinion, Ratatosk, your pulmonologist is not giving you good advice. A lot of CFers play hockey and swim. Gunnar Esiason, one of the more famous CFers, played football and plays hockey now. Perhaps many CF patients are too small to play football (I definitely was), but in my mind, that would be the only reason for them to not play.
Aboveallislove - my center never gave me advice on specific activities. I don't know how old your child is, but I'd say that any activity that gets their lungs working and that they enjoy is probably great. And the enjoyment part is so important! If they get bored on the treadmill, maybe they'd like the elliptical or rowing machine. Or maybe those are boring too, but they like jumping rope. Maybe a team sport is more their speed. Any of it is good!
As kids get older, being active may be even more important to help counteract disease progression. I am in the midst of a battle with M. Abscessus, and despite the fact that I've been smear positive now for almost 8 years, I'm still thriving and my CT scans show very little progression over that time. And I believe it's because I exercise almost every day. Some days I only have time to jump on the elliptical for 20-30 minutes. Other days I run in the morning and lift weights during lunch. I have a much easier time clearing out my lungs when I'm active - my airway clearance is much more productive. If I take just a few days off, I can tell - my mucus is darker, thicker, and harder to get out. Living an active lifestyle is so important for us.
Sorry if I'm rambling too much. I'm a HUGE exercise advocate!