Nanny and boss romance novels are appealing because they offer a unique blend of emotions. There's the initial awkwardness of the professional relationship which gradually transforms into something more. The characters often have to overcome their own insecurities and the opinions of others. The boss may worry about his reputation and the nanny might fear losing her job. But through it all, the development of their love story, from the first shy glances to the passionate confessions, makes for a very engaging read.
One reason is the forbidden - love aspect. Since the nanny and the boss are in a professional relationship, falling in love goes against the norms of that relationship. This adds a sense of excitement and drama. Also, the chance to see how two people from different walks of life, with different responsibilities, can find common ground and love is really appealing. For example, the nanny might be focused on family - like care while the boss is all about business, but they still manage to connect on a deeper level.