Education Secretary William J. Bennett testified yesterday before a House committee on the Reagan administration's proposal to convert federal school aid for low-income children into a voucher plan, describing the measure as a way to expand opportunities for students who are "captive to a poor education."

But the voucher proposal received a generally chilly reception from both Democrats and Republicans on the House Education and Labor Committee.

Under the plan, unveiled last November, the 20-year-old Chapter One program, which now provides $3.2 billion in remedial education aid to about 4.8 million low-income children, would be transformed into a voucher system by which parents, not school officials, would decide where funds are spent.

Parents could use their vouchers to pay for tuition or special instruction at private schools, at the public schools such children attend, or at public schools outside their attendance zone or school district.

At yesterday's hearing on the proposal, called The Equity and Choice Act, or TEACH, committee chairman Augustus F. Hawkins (D-Calif.) labeled the plan "irresponsible" and said it would allow private schools to engage in "unfair competition."

Rep. William F. Goodling of Pennsylvania, the committee's ranking Republican, said the measure would be "teasing the poor" because the administration has proposed no increase in federal aid.

Goodling, a former high school principal, said the plan would hurt the public schools that children leave and would create "a real management problem for administrators." Bennett responded that "lots of [school] administrators . . . [are] right to be uneasy. The American people are saying they are not doing a good job." He said he rejected "the notion that we need a place to dump children because bad teachers should get students, too."

The amount available under the voucher plan would average about $610 per child a year, varying according to Chapter One spending in particular school districts or states. Last year, spending was $1,002 per child in the District of Columbia, $791 in Maryland, and $686 in Virginia. Bennett said the money would provide "meaningful choice" for a substantial number of low-income children, since the median tuition at private elementary schools this year is $773. According to U.S. Census Bureau estimates, the median is $695 at church-related schools, most of them Catholic, which about 85 percent of private school youngsters attend.

About 225,000 recipients of Chapter One funds attend private schools and get remedial instruction from public school employes.