I agree with everyone here. While I don't have CF, I've got bronchiectasis which requires the same amounts of treatments pretty much as it affects the lungs.
I was a lot like Melissa (Jazzysmom). When i first got diagnosed I rarely did my physio and all that. I was supposed to do it 3 times a day and was lucky if i did it once a week. I always took my meds mind you. But even still, by just not doing physio, i sit here 4 years later, with a lung function of 27% and in need of a tx. All that time i could have been clearing my chest and preventing damage and I didn't, b/c i didn't feel like I needed it. I'm not saying that solely b/c I neglected to do physio strictly, that that's why my lungs are so bad. Either way, needing a tx was probably inevitable for me. But my point is, had i been complient, it could have delayed it a lot longer.
Just b/c your child doesn't look sick, or doesn't act it, or what not, the bottom line is, your child has a disease that needs to be treated and right now the only person who can do that for them, is you. I sincerely hope that for the sake of your child you begin doing them regularly. Silent damage is the worst kind there is.