The diagnosis of schizophrenia is typically quite unique from that of bipolar with or without autism. The differentiation is best made when reviewing the history of childhood symptoms. If there are major impairments with social reciprocity, language and the presence of stereotypic behaviors in childhood then the diagnosis of autism may be further explored. Most children with schizophrenia diagnosed as an adult can recall some childhood symptoms, but, they are not typically as impairing as Autism is in youth.
Childhood schizophrenia is typically differentiated from Autism in youth by the presence of auditory hallucinations, visual hallucinations and paranoid delusions. A family history of schizophrenia may also lend credence toward a diagnosis. Children with Autism may seem to respond to internal stimuli (outward signs may be speaking to themselves or acting as if there is something or someone else present), as children with schizophrenia may do, however, this is typically in the context of stress or anxiety (such as a change in a routine or when faced with a non-preferred activity).
Bipolar disorder, when it presents in the under 18 population, typically occurs later in adolescence (13–17). In a child with Autism, the classical symptoms of social interaction and communication deficits and restricted repetitive movements must be present within the early developmental period. In your question, then you are thinking about at a child with a history of impairment typical of Autism who then experiences a manic episode (needed to define bipolar from unipolar depression). I have worked with children that have Autism who stop sleeping and go on to develop cardinal symptoms of mania such as : grandiosity, pressured speech and euphoria or irritability. These children typically have a family history of Bipolar I disorder.
An evaluation by a child psychiatrist is recommended to determine the diagnosis. It may take several sessions or perhaps several months to fully diagnose this condition.