I enjoyed the book, but I don't think a "young" 11 year old would enjoy reading about unloved children. The "poor little rich kid" in the story is really heartbreaking.
A lot would go over her head, but it's pretty much totally an adult take on dysfunctional parenting/employment. I don't see what attraction it would hold for your daughter.
The nanny knows the boss husband is cheating and keeps it from the wife and wonders about the ethics of it. In fact, the book is chock full of ethical issues well above the average 11 year old's head.