If you don't won't to keep her and don't need the money, politely tell the parents they need to find someone else to keep her. Then you won't have to worry about it....
Sounds like she is just having a hard time getting over a chest cold or it could be allergy related. Ask the parents if they have been to the doctor or ask that they go see one. Make sure everyone is washing their hands and use sanitizers on all handles in the house and toys.
Sounds like your...