Hi Theresa,
Not all children with ADHD end up having bipolar as adults, but most will continue to have ADHD as adults. Both conditions have strong genetic links. That means that if a child has a parent who has bipolar, there is an increased risk that the child might develop bipolar disease too, at some point. The same can be said of adults with ADHD- their offspring are at higher risk of also having ADHD.
Note that some kids diagnosed with ADHD actually have bipolar because some of the symptoms overlap. The same can be said of some children diagnosed with bipolar who might actually have ADHD. And to complicate things, kids can have both ADHD *and* bipolar.
There's a whole area here at Health Central with a lot of information and resources on Bipolar Disorder at http://www.healthcentral.com/bipolar/ . I'm sure you'll find lots of answers to your questions about bipolar there.
Terry