This is a great idea for a Disney franchise or series, but as a stand-alone film it falls a little flat. Though a narrator tells us this isn't your typical "princess story," Secret Society of Second-Born Royals does follow what feels like a formula of rebellious teen turned empowered superhero, complete with princess status (and ball gown). The diverse cast's teen actors are charismatic enough, but the script paints their characters in very broad strokes, relying on some superficial stereotypes to give them personalities (shallow Instagrammer, popular party boy, invisible guy, rebellious teen, etc.) and life problems that are too easily resolved. A key bonding scene, where the teens open up to each other, comes too soon, and an awkward "day off" montage later seems to want to compensate for that. This, along with other questionable decisions concerning the faux European setting and a mixed bag of accents, makes this film more likely to find an audience with younger viewers than with teens.
What might go over their heads, however, is the theme of questioning the validity of monarchies in contemporary societies. There are also some references to real life that younger viewers might not catch, including portraits of actual second-born royalty in a palace hall. One is of England's Prince Harry, who -- in a curious life-imitating-art twist -- renounced his royal duties.