Holed up in windowless hotel conference rooms near Washington, D.C., scientists have been busy rewriting the bible of American mental illness.

It is the first revision of the nearly 1,000-page tome in 15 years, and one of the top priorities of the insular conclave is to rethink some children's disorders, particularly bipolar disorder. The fear is that too many treatable children are slipping between the cracks, either because of misdiagnosis or—more controversially—because they...