Medicaid Child Health Amendments of 1991 - Amends title XIX (Medicaid) of the Social Security Act to permit States to provide Medicaid coverage to all children who have attained age six but not age 19 and whose family income does not exceed 185 percent of the Federal poverty level. (Currently, States are required to cover only certain children between such ages, and only those whose family income does not exceed 133 percent of the Federal poverty level.)

Permits States to provide an additional 12 months (currently, 12 months total) of extended Medicaid coverage to families whose AFDC eligibility ends due to the caretaker relative's employment.

Requires States which provide prospective Medicaid payments to hospitals to reimburse hospitals which serve a disproportionate share of low-income patients for exceptionally costly or lengthy stays by children up to age 19.

Directs States to provide Medicaid coverage for disabled children under age 19 but who are not eligible for benefits under title XVI (Supplemental Security Income) of the Social Security Act.

Prohibits the discontinuance of the Medicaid benefits of a child under age 19 until he or she is determined to be ineligible for such benefits on any basis.

Allows States to extend Medicaid coverage to: (1) foster care children under age 19 whose incomes are above State cash assistance eligibility levels but below the Federal poverty level; and (2) migratory agricultural workers and their families.