Does your child take probiotics? Be sure all yeast is gone before taking GSH. GSH may have highlighted the yeast problem? Doctors Data has a stool analysis test that I think is very helpful. You can see good & bad bacteria levels, digestion issues: digestion, absorption, inflammation markers. We do the test just to make sure the gut is in balance. The GI is grossly overlooked at the clinics because the Dr.s (ours anyway) are all pulmonologists. The GI is very important - a healthy gut is proven to have a positive affect on lung health.
We take liposomal GSH. Liposomal form allow the GSH to get into the red blood cells. CFers have GSH deficiency so don't lose faith in GSH it is critically important. (My Dr. has been researching GSH for 10 years-- )Wellness Pharmacy can help you find a Dr. in your area that knows about GSH.
I believe there were earlier posts related to GSH and yeast on the other board too.

http://www.wellnesshealth.com/default.asp?id=5

Aliment Pharmacol Ther. 2004 Oct 1;20(7):813-9.

Intestinal inflammation is a frequent feature of cystic fibrosis and is reduced
by probiotic administration.

Bruzzese E, Raia V, Gaudiello G, Polito G, Buccigrossi V, Formicola V, Guarino
A.
CONCLUSIONS:
Intestinal inflammation is a major feature of cystic fibrosis and is reduced by
probiotics. The latter finding suggests that intestinal microflora play a major
role in intestinal inflammation in cystic fibrosis children.

My child takes Klaire Labs "Complete"