It does say in text that she mentally regressed to being a child as well, which they give an example through her bring frustrated that she couldn't communicate why she doesn't want to marry, and instead yelled like she was having a tantrum and ran off because children have difficulties explaining their own feelings and have more emotional regulation issues.
Her brain wouldn't be fully developed so she literally is acting like a child because she is one, she has the literal brain of a child.
Having knowledge/memories of something isn't the same as being the actual age of those memories.
Honestly, I wish more regression/reincarnation stories did this more often because it makes biological sense and bc it's less weird for romances, especially for stories where they just suddenly start acting like full ass adults but in a child body.
I’m only on chapter 2 but idk how I feel about her personality… she clearly retains her memories of her past life so how is she gonna act like a child is eye candy