The law specifies that employer-sponsored insurance is not affordable if a worker’s share of the premium is more than 9.5 percent of the worker’s household income. The I.R.S. said this calculation should be based solely on the cost of individual coverage, what the worker would pay for “self-only coverage.”

“This is bad news for kids,” said Jocelyn A. Guyer, an executive director of the Center for Children and Families at Georgetown University. “We can see kids falling through the cracks. They will lack access to affordable employer-based family coverage and still be locked out of tax credits to help them buy coverage for their kids in the marketplaces, or exchanges, being established in every state.”

In 2012, according to an annual survey by the Kaiser Family Foundation, total premiums for employer-sponsored health insurance averaged $5,615 a year for single coverage and $15,745 for family coverage. The employee’s share of the premium averaged $951 for individual coverage and more than four times as much, $4,316, for family coverage.

Under the I.R.S. rule, such costs would be considered affordable for a family making $35,000 a year, even though the family would have to spend 12 percent of its income for full coverage under the employer’s plan.

The tax agency proposed this approach in August 2011 and made no change in the definition of “affordable coverage” despite protests from advocates for children and low-income people and many employers. Employers did not want to be required to pay for coverage of employees’ dependents. But they said that family members should have access to subsidies so they could buy insurance on their own.