For millions of American parents and grandparents, “Ms. Rachel” is not just a celebrity or social media influencer, but a daily presence in their living rooms. Her voice plays on TVs, tablets, and phones during a child’s earliest years as part of her educational programming for toddlers. Her real name is Rachel Griffin Accurso, the face behind the viral YouTube series “Songs for Littles.” Over the past six years, Accurso has grown the brand from homemade videos into a multimillion-dollar children’s entertainment operation focused on early language development for babies and toddlers. As her audience and influence have grown, Accurso...