I am sorry. But, I don't see a way for her to do this legally.
Normally I would suggest that she participate in a J-1 nanny/au pair program. But, I believe that she is too old for most of the programs.
If at all possible, try to convince her not to work for cash and off-the-books.
It might be possible for her to get a visa to come and 'intern' at a Montessori school in the US. But, she couldn't be a nanny.
PS For more information on her options/limitations, please consider a private consultation with an attorney. Most of us can do Face-to-Face 3-way Skype calls.
PPS .... she could also explore the possibility of getting a higher degree in child/clinical psychology and working at a day-care center that is run by the University.