I see your point, but i still think Ai should turn back to an adult. It doesn't matter she's Ai Haibara or Shiho Miyano, they're just a same person, but Ai Haibara is a body of a kid and Shiho Miyano is a body of a adult, while she is an adult, that why Shiho Miyano fit her more. I want her to be herself, imagine an adult who has to live in a body of a child, she has the taste of a adult, she's not interested in kid's interests, how do you expect her to enjoy her life. She's still young, she doesn't need to be a kid to learn new things, to experience 'childhood' that you talked about, she can go back as Shiho Miyano to do it.