cant remember the name of the series but another isekai had a really good reason for why the MC started acting more kid like as the series went on, that while the soul might be an adult hes in the body of a child the soul would over time adjust to better "fit" the current body, not worded the best as im tryna explain it from a series i red a few years back, but i always liked it and wished more series would do similar. and is basically what my head canon is, she acts like a child because she is a child and her sould is gradually changing to fit it better