When I was in training in the late 1980's, the concept of Bipolar illness in Children was unheard of. It was supposed to occur in late adolescence and early adulthood and so we "didn't see it' in younger children. It came as a surprise when I saw an 8 year old with classic, undeniable symptoms that "walked like a duck, looked like a duck and quacked like a duck". The fact that bipolar occurs in children makes sense because when adults with legitimate diagnosis of Bipolar are interviewed they typically tell you that they date back their symptoms to childhood. Often times they describe years of misdiagnosis until it finally makes sense.
We used to think that 10% of ADHD kids would "convert" to Bipolar but in reality the "ADHD symptoms" were probably an early manifestation of Bipolar.
We psychiatrists are at a disadvantage in that we do not have solid neurologic or blood testing that can give us a definitive diagnosis, instead, we have to base our hunches on history provided by parents and patients and trying to put the "pieces of the puzzle" together, eg, family history of Bipolar, response to previous medication, etc.
Websites such as www.bpkids.org are excellent as parent resources.