HARRISBURG, — Gov. Tom Corbett’s administration is being told it must shift tens of thousands of Pennsylvania children from a state-subsidized health insurance program to Medicaid, although the Republican governor is not saying yet whether he will comply with the federal directive.

U.S. Health and Human Services Secretary Kathleen Sebelius told Corbett in a Friday letter the switch is required by the 2010 federal health care law and it will simplify coverage for families by aligning children under the same program as their parents.

Corbett has protested that some children will have to find new doctors, but Sebelius said many other states have already prepared for the transition by eliminating age-based eligibility rules for Medicaid coverage and by extending Medicaid eligibility to children 6 or older whose families earn up to 133 percent of the federal poverty level. Advocates for the poor say children are better off under Medicaid anyway because its coverage is more comprehensive than Pennsylvania’s Children’s Health Insurance Program, or CHIP.

Corbett’s top insurance regulator, Michael Consedine, on Tuesday called Sebelius’ response “disappointing” and could not yet say whether the administration will comply with it or seek to block it, perhaps through a court challenge.

“I don’t think the governor’s ready to give up the fight for this program, so we continue to evaluate our options,” Consedine told The Associated Press.

Currently, Pennsylvania’s Medicaid program covers children younger than 6 whose families earn up to 133 percent of the federal poverty level, and children 6 and older whose families earn up to 100 percent of the federal poverty level — about $23,550 for a family of four this year. The 2010 federal health care law also expands Medicaid to working-age adults who earn up to 133 percent of the federal poverty level, although Corbett has said he will not allow Pennsylvania to participate without certain cost-savings concessions from the federal government.

The change for children takes effect Jan. 1, although Sebelius offered in her letter to cooperate in a phased-in transition. She also suggested that Pennsylvania can structure its child health care to make two programs appear more like one that resembles CHIP, with different plan benefits and cost-sharing obligations for some families.

Consedine said the administration will seek more information about those ideas.

He estimated the shift would affect 50,000 children 6 and older.