I'm not sure about the film. Throughout the film, Johansson links the incidents she observes with a monologue to explain what's going on. I don't think works. She comes across as distant and not wholly involved (in what's going on around her) in the monologues (which is something I think most people that age are not), which is contrasted by what happens in the scenes where she appears to be intently interested in the boy she cares for.
In addition I'm not sure about Johansson in the role. Whether or not it's the script I don't know, but I get the sense that the Nanny is supposed to be a borderline nerdy geek type who falls into the job rather than go to graduate school and Johansson doesn't really pull that off.
I don't know whether my doubts about Johansson are because of the script, the original story (I don't know if it was written as a faux anthroprological research report, but if it was, it doesn't always work on screen) or Johansson, but I don't think it works as it is now