While I have no first-hand experience in this, I did know a Catholic family (whose daughter went with mine to separate (Catholic) school) who had a JW live-in nanny. Since I was a JW and this nanny had no transportation, we would sometimes bring her to the meetings with us. She preached to the children. This I know, because the children knew a great deal about the JW religion and could empathize with my daughter.
I would say that E-man has it just about right -- ANY Witness nanny would find it difficult not to drop little informational tidbits of 'the good news' to the family s/he worked with. After all, nannies grow attached to their charges, and would not wish for them to die at Armageddon!! If s/he 'saves' the children, then, they, in turn, may eventually 'save' their opposed parents and Jehovah knows how many!
Children of Jehovah's Witnesses are psychologically harmed -- my son grew up terrified of perishing in one of the earthquakes that would precede Armageddon, for example -- do you really want this for your children?
JW Nannies would be trustworthy, kind and diligent, if they are good dubbies, but would, I think, find it difficult to keep silent about their beliefs because of the sheer amount of time they would spend with a family should they "live-in".
outnfree
When a true genius appears in the world, you may know him by this sign, that the dunces are all in confederacy against him. -- Jonathan Swift