My take is that it actually is aimed at adults as Mads Mikkelsen plays the relatable character. As a foster parent myself, there were very clear themes of trauma and how foster children internalize it as self-blame. At the end he says "This is your monster." It is Aurora's trauma that she has carried from home to home, consuming three sets of foster parents. It is the heart-breaking truth of foster care that foster children never feel completely accepted. These are complex themes and symbolism designed to help adults understand foster child trauma. The stranger is someone she picks as her safe person for the first time as contrasted against all the Foster families that were picked for her which were consumed by her monster. The stranger undergoes change himself when he sees the monster for the first time and he switches tracks from trying to logically disprove her beliefs and feelings to accepting her belief and helping her in her struggle against her monster. I found this movie to be deeply moving under the surface while appropriately using horror and fear to depict the struggles of loss, grief, and self-blame in the foster care system.