You're absolutely correct in your observations, as all the detritus from the bunnies can potentially cause problems in healthy people, and the risk is multiplied in those with CF, especially very small ones.
Now then: fastidious maintainance of the bunny room could very well minimize the risk to everyone involved, but this is the problem, at least according to what I infer from your post. And knowing that people who do not tend to domestic animals properly usually are more willing to give them up than get with the clean program and stay with it day in and day out, this is probably the best approach, if followed up with a very thorough cleaning of the room. It is actually possible, in many cases, to keep animals in situations, but not if they're being kept this way. It's also not goof for the rabbits, who, while not immediately endangered nor human, do also deserve some consideration. All in all, this is a real mess, and especially a problem with the small one with CF being exposed to this. Your conclusions about various bunny detritus and its spread throughout the dwelling is correct, although, again, this could be minimized with proper animal care, which seemingly is not being given. This is a serious concern, and may well need to be brought to the attention of child protection services, who could assess this objectively. However, this would no doubt cause a painful rift. It might be brought up as a veiled threat, and might get the attention of the parents, but it's still likely to cause family strain at least. It's difficult enough just being a grandparent and trying to pass along helpful insights without this very complex additional situation.
I hope this is helpful to you. Good luck with this, and please follow up with us here as needed.
John Kenyon, EMT, CCT
Non-invasive cardiology tech, Emergency and Critical Care technician, Critical Incident Stress Mgmt. specialist