Sen. Lloyd Bentsen (D-Tex.) introduced legislation that would expand Medicaid health-care coverage to thousands of low-income children and pregnant women at an estimated annual cost of $1 billion. Bentsen, chairman of the Senate Finance Committee, is attempting to provide Medicaid coverage to pregnant women and to children under age 6 in families whose income is 185% of the poverty level--$22,385 for a family of four. Under legislation enacted last year and being phased in during fiscal 1989 and 1990, Medicaid coverage is guaranteed to pregnant women and infants under age 1 in families whose income is at the poverty level--$12,100 for a family of four. Bentsen's bill would expand coverage to an additional 300,000 pregnant women and to 1.6 million children under age 6 in fiscal years 1990 and 1991, his office said.