Legislation to subsidize child care expenses for low-income Americans and expand and improve day care facilities for all families cleared a Senate committee. The $2.5-billion-a-year measure, approved by the Senate Labor and Human Resources Committee on a voice vote, now goes to the full Senate, probably in September. A similar measure is pending in the House Education and Labor Committee. The bill, sponsored by Sen. Christopher J. Dodd (D-Conn.), would offer child care subsidies for families at or below the median state income level, which varies greatly from state to state but averages about $32,000 nationwide. It would also provide seed money to states to expand child care services, establish guidelines for training, safety and classroom size and improve enforcement mechanisms.