What seems to be the problem, faced by the main characters in the perfect nanny?

Paul and Myriam are a perfect couple with two perfectly sweet children: Mila and Adam. When Myriam begins to get bored with her perfect life as a stay-at-home mother, she decides that she wants to go back to work as a lawyer. The couple hires a nanny who seems to be wonderful in every way: she cleans their house, cooks their meals, and cares for their children. But as time goes by, they slowly start to realize that there is something very wrong with their nanny who eventually murders their children. The novel explores motherhood, careers, the class divide, mental health, childhood, and greed.