As far as I remember Aryel is also a reincarnation rather than a transmigration. She IS Arelyn for all intents and purposes. She was born as her, lived as her and at some point regained her memories of her last life. In this case it makes more sense that she acts like a child because she is one. She just has memories that extend beyond her lived experience. So she might be mentally a bit older ~8 or 9? But at the end of the day she’s just a kid with a past life as a source of knowledge.
Abuse can have you regress your age mentally despite that there was a point in your life that your abuser love you, doesn't necessarily mean that when she sees another caregiver they want to what they used to have with the abuser but essentially another safe person. You can tell she sees him differently.

gonna drop this because i saw a comment saying it’s weird for arelyn to feel that way toward pession.
i understand that arelyn has an adult consciousness. but we also can’t ignore the fact that she’s still living in a child’s body, with a child’s brain that isn’t fully developed yet. her maturity mainly comes from her memories and past experiences, but those aren’t physical. they don’t automatically override the biological reality of her current body.
so realistically, her adult memories would mix with her younger brain’s emotions and instincts. meaning, it makes sense that she would sometimes think, react, or feel like a kid, even if parts of her adult mindset still show up and interfere.