The ending of the requirement to preserve coverage has already proved catastrophic for low-income families and children. At least 1.1 million children are believed to have lost Medicaid coverage since the policy ended, according to data from 15 states analyzed by KFF.

Some states have not yet published data breaking down coverage losses by age, giving researchers a limited view of the toll on children.

Children have higher, or more generous, eligibility limits for enrollment in Medicaid and CHIP, and so they were expected to remain on the rolls in greater numbers. Public health experts have feared for weeks that the coverage losses were the result of errors by state Medicaid agencies.

Joan Alker, the executive director of the Georgetown center, said that children are on average eligible up to 2.5 times the federal poverty level through Medicaid or CHIP. When they lose coverage, she added, there is often nowhere else to look for health insurance.

“Children are not expensive to cover, but they’re regular utilizers of care,” she said. “They frequently have ear infections, asthma, things that are very treatable but require them to have access to care.” Gaps in coverage, Ms. Alker added, can be life-threatening.