Welcome! I just want to clarify, we use "psychiatrist" to mean "psychiatrist". I'm wondering if you meant it to mean a psychologist or other type of professional? I ask because you refer to a dr as prescribing medications, whereas if a psychiatrist were involved, they would typically be prescribing this type of medication.
Also, you want to make sure that you have received as thorough of an evaluation as possible to get the diagnosis before going the bipolar route and completely ruling out other diagnosis's that would be treated entirely differently. So, what types of profs have been involved in diagnosing your child? Has she had neuropsycholgical testing? Has she been evaluated by a child & adolescent certified psychiatrist who also reviewed the testing results? Although there is no "test" for bipolar, the testing can rule out other diagnosis's, find comorbid disorders, reveal deficits in areas that are common for bipolar, and reveal difficulties in areas that need supports at school.
If you are comfortable with the bipolar diagnosis, lithium is a fine place to start, at least with older kids. My only question would be is it safe for a child that young and I'm really not sure. It would probably be ok after blood tests are done, but of course, I'm not a dr.
Here are links to web sites that I have found very useful:
http://www.dbsalliance.org/
http://www.jbrf.org/
There are many books about kids with bipolar- others can recommend what they have found most useful.
Again, welcome!! The medication battle can be long and strenuous sometimes, but lithium, along with a few others (lamictal for instance) is tried and true in the world of BiPolar (BP), but I tend to hear about anti-pshotics (AP's) being used instead of them in younger children a lot. But, as I suggested before, honing in on the diagnosis as much as possible and YOU being comfortable with it is a big first step. Also, I'd start the IEP process now if you haven't done this already.