The nanny is surely one of the main characters in the 'Nanny Dearest True Story'. Depending on the nature of the story, if it's about her relationship with the family, then the mother and father in the family are main characters. They would have hired her, set the rules for her employment, etc. The children are also main characters as the nanny's role is mainly centered around taking care of them. There could also be neighbors or friends of the family who interacted with the nanny and influenced the events of the story.
In the 'Nanny Dearest True Story', the central figure would be the nanny, of course. She could be someone with a unique personality. Then there would be the family she served. The parents might be important characters as they hired the nanny. And the children in the family are also likely to be key characters, as the nanny's relationship with them would be a big part of the story. Maybe there were also other household staff or relatives who could play secondary but still significant roles.