Children's Mental Health Accessibility Act of 2012 - Amends title XIX (Medicaid) of the Social Security Act to: (1) expand the Medicaid home and community-based services waiver to include youth in need of services provided in a psychiatric residential treatment facility; (2) limit, under the waiver, the total number of Medicaid inpatient bed days at psychiatric residential treatment facilities during each fiscal year to the total number for the previous fiscal year as increased by the estimated percentage increase (if any) in the population of individuals under 21 residing in the state over the preceding 12-month period; and (3) change references to mental retardation to references to an intellectual disability.