I was diagnosed with bipolar at 17 and my mom claimed to doctors that I was manic even as a child. Even though she knew full well that I was diagnosed with ADHD at that age. I'm starting to think that diagnosis is more accurate.

Here are the articles that led me to that conclusion, it talks about the difference between ADHD and bipolar:

Is It Bipolar Disorder or ADHD? Symptoms and Treatment of Bipolar Disorder | ADDitude - Attention Deficit Information & Resources

ADHD and Bipolar: A Comparison - Bipolar - ADHD

Keep in mind that ADHD is vastly more common than childhood bipolar, so balance of probabilities would suggest the former diagnosis is correct, but I don't know your child personally, so I can't say.