If the mom is at home and so terrified of germs they should be keeping the children home with them. If they are still working then they need to realize that the chances of them coming into contact with something "serious" is more likely to happen at work or on the bus going to and from work or the grocery store or the doctor's office getting her child immunized against everything going or whatever. The child may come in contact with germs but in the grand scheme of things the chances of it being something that will be a danger to her is actually quite remote.

The caregiver has a responsibility and a right to conduct her program any way she wishes and it is the parent's responsibility to accept it or to move on. No way should the caregiver be forced to change. Now having said that there may be times when rethinking might be a good idea - deciding if that trip to the pond to play with frogs and turtles is necessary or at least giving the parent a heads up to plan to keep their child home that day.