I'm a pediatric psychotherapist, and a mother of a toddler. This is downright scary! I have worked in several extreme settings (locked hospitals, level 5 schools, etc.) and I have yet to see a child this young that I would diagnose with bi-polar disorder.

I have to say that I'm very surprised that an article on this topic wasn't written sooner. I've been doing some personal research on this topic for about 2 years.  What doesn't surprise me is the lack of responses to this great article.
I was attending university, studying neurological sciences when I was diagnosed with bipolar disorder.  After getting my illness under control, I went back to school and wound up in a child based field.  My education for this field included brain development for children ages birth to 5 years old.  Knowing what I know from child development classes, and neurological sciences, all I can say is that people in the business of making and pushing 'legal' drugs of this nature for children disgust me. 
I'm not an expert, but I believe 100% that bipolar disorder does not happen this early on in life.  If you've met a toddler, or even a preschool aged child, you know they have mood swings...a LOT of mood swings.  There are so many reasons for their mood swings; they can't communicate their needs, they're hungry or tired, need extra attention, their environment, etc.  The list could go on forever.
I did work with one boy who was around 2 1/2 years old when he was diagnosed with bipolar disorder.  When I arrived at work and heard the meds he was on, and that he had been diagnosed with this illness, I was beyond horrified and disgusted.  What disgusted me more was his mother for letting him take these drugs.  Prior to these meds he was pretty much a very bright, outgoing, high energy child.  He was not easily angered,  but when he was he would become violent towards himself.  His mother failed to see that this boy was learning his response to anger from his older brother, not because he had other issues.  After the meds, he became a very placid boy.  He would stare off into space frequently, and you'd have to call his name several times to even get a response.  It was so sad.
When society sees what's happened to teenagers who take antidepressants, and other similar drugs, how can doctors diagnose, and parents give meds to very young children?  Especially, when their brain is so delicate and developing so quickly.  The research for teens who have taken these sort of drugs is just beginning to come out, not including the suicides we have heard about in he media.  We WILL eventually see how the young brain has handled these types of treatments in 10-20 years, and to be honest, I'm afraid.