It is not bad for you to bring your neice to the CF walks, as long as she doesn't culture B. Cepacia. That if the bug that they ask you not to come if you have since they assume that people with CF WILL be at the walk. B. Cepacia is a very bad bacteria to get, so if that is an issue, then you wouldn't want to spread it. Most CF centers in the US would like you to keep kids with CF away from other CF kids, so they do not pass bugs back and forth. However if they culture the same things then they are pretty lenient about it.
The best bet is to keep the "3 ft. rule" just keep the kids with CF 3 feet awy from eachother. That should keep them from passing bacterias back and forth. And being out in the open is better than being in the same clinic together anyway!
mom of Rachel 5 with CF and a boy on the way, no CF
Andrea, mom of Rachel, nearly 5 with CF