It is tricky, as noted above, because you don't want to see your child infected. However, the nature of CF is chronic lung infection, so there is nothing you can do. Staph is everywhere and I describe my lungs (and other CF lungs) as fly paper. If it's around today I will colonize it tomorrow. It is also true that colonization doesn't always mean infection. I colonize MRSA with every visit, hence, every day. I need antibiotic help about 4-5 times a year. I am lucky that neither myself nor my physician have abused antibiotics, and at 41 I still get results with orals. I don't even worry about the day that I need i.v. antibiotics since they are getting better and better.
Also, there is some merit to the concept that staph seems to rule the playground and chases away pseudomonas. Doesn't mean the two bugs can't coexist, but focus on the lack of pseudomonas on current cultures.