Such a Fun Age by Kiley Reid

Alix Chamberlain is a wealthy blogger and public speaker in her early thirties who has built a brand known as "LetHer Speak" around the practice of writing old-fashioned letters to businesses, often in exchange for free product samples, and encouraging women to be assertive. Alix's family has moved from New York City to her hometown of Philadelphia for her husband Peter's job as a television anchor, and her career is stalling as she raises two children and attempts to write her first book. Alix hires Emira Tucker, a 25-year-old African-American college graduate, as a babysitter to care for her three-year-old daughter Briar. Alix also has an infant daughter named Catherine.