In my opinion, the main reason why children request the same story every single night, sometimes multiple times a night is that kids experience heightened levels of the stress hormone cortisol in response to new and potentially frightening situations, whereas everything familiar makes them feel comfortable. They don't feel frightened. And their cortisol levels decrease. So children enjoy the familiar 'cause this makes them feel relaxed. However, younger children need more time to encode information than older children, and they forget faster. That's why they need repetition to make the strange familiar. (You can find the video featuring this answer on our YouTube channel called IELTS Speaking Assistant, playlist SAMPLE ANSWERS)