As others have said, its incredibly common, although rarely discussed, in children.
If it helps, a friend of mine who actually does have schizophrenia (developed in his mid 20s) and now works with people who have recently been diagnosed told me that part of their training teaches them, that a significant proportion of the population hear voices at some time in their lives. Its incredibly common in children, especially when they are feeling anxious for some reason. Of all the people who seek help for hearing voices, only around 1% are actually suffering from schizophrenia/psychosis.
The difference is that people which schizophrenia/psychosis are unable to differentiate their thoughs/voices from reality.
I got the following description of schizophrenia from the MIND website:
What is schizophrenia?
The term schizophrenia is widely used in the mental health system. Doctors may describe it as a psychosis. They mean that, in their view, a person can't distinguish their own intense thoughts, ideas, perceptions and imaginings from reality (the shared perceptions, sets of ideas and values that other people in that culture hold to be real). Among other symptoms, a person might be hearing voices, or may believe that other people can read their mind and control their thoughts.
In the vast majority of cases, children who hear voices are struggling to cope with some perceived source of stress or are having problems learning to deal with the strength of their own emotions. In most cases, the situation resolves either with some help with learning to manage their stress levels or regulate their emotions or even just in line with development of their emotional maturity.
From what I've read online, I think many of the children who start to hear voices are simply becoming aware of and starting to wrestle with their own consciences. I do wonder whether it depends on whether they are visual or auditory thinkers, as obviously auditory thinkers are more likely to perceive and battle with their conscience in a verbal rather than instinctively emotive way, iyswim.
I also think gentler children, who are by nature more likely to try and suppress unkind or (their perception of) bad thoughts are possibly more likely to hear 'bad' voices at some point. Again, auditory thinkers would be more likely to have their suppressed thoughts come out in an auditory way, rather than a visual child who might be more prone to nightmares.