There was a thread on here a few weeks back of adults doing this . Seems very common when people are on their own doing a routine type task (seemed common theme to be when people were cooking). It’s referred to as “self explaining “ sometimes in psychology.
I guess the only difference is your DC hasn’t yet twigged that people will think she is odd if she does this in front of other people.as adults, despite it being so common we all tend to hide it from others for fear of being labelled a bit “mad” .
my Ex had psychotic illness. He heard voices. The cause of hearing voices is unknown but we do know that when under a scan “normally “ different parts of the brain are activated when someone is using their inner voice (eg voice in your head) versus an external noise. In someone hearing voices both parts of the brain are active- they literally hear their inner voice as an external noise. What is interesting is that my Ex’s voice were at their most benign “observational” - he had a couple of people just constantly observing what he was doing and making commentary on it . Often they’d kick off when he was doing a task like gardening, cooking. Clearly he was doing this self explaining but he couldn’t recognise it was his inner voice and he heard it as an external conversation from others in the room. His voices would then get more critical and then abusive the more stress and fear he felt.
so, for my Ex , what seems to be benign in a lot of people in terms of this self explaining, was turned into hallucinations because of this “cross wiring” of different parts of the brain in response to the stimulus
clearly as humans, this self explaining is common and must provide a certain function for the brain. Some research suggests it aids emotional control, others that it is helping work through problems, . Just like dreaming and it’s role in processing our emotions, feeling etc for events that have happened. We’re only just beginning to understand this with the use of therapies such as EMDR
at an extreme level of talking out loud and self narrating , there is maladaptive daydreaming - again threads on here in the past indicate it isn’t that unusual. In a lot of cases people keep it mostly under control . But very few people will seek help or acknowledge it except on anon MN posts