And that, Diel believes, could add roughly $40 million to the health care department’s projection.

Unlike the regular Medi-Cal program, which is a partnership with the federal government, the state of California must foot the full bill for coverage of kids who are not in the country legally, except when they seek emergency services.

What’s missing from the state’s estimate of that bill, Diehl said, are at least 70,000 undocumented children currently receiving benefits through Kaiser Permanente and about 20,000 more who get varying levels of care through county programs.

However, those additional 90,000 children will not be as expensive to cover as the ones counted by state health officials, because they already are receiving care.

Most of the 170,000 eligible children included in the state’s estimate “don’t have any care at all right now, outside of some of them getting care in the ER,” Diel explained. “It will be their first year of coverage, and that’s a lot more expensive.”

The 90,000 additional children who are already getting care would cost about half as much as the first-timers, on average, because “it’s easier to care for them,” he said.

But the state’s eligibility numbers could be off by even more than 90,000, said Richard Figueroa, program director at the California Endowment, a health care philanthropy.

He said as many as 300,000 children in the country illegally might be eligible for full Medi-Cal benefits, though no one really knows exactly how many children in California are without legal papers.

Diel said his estimate of $40 million in extra state spending is a conservative number. “It’s speculation, but it’s speculation based on past experience,” he said.

Ronald Coleman, government affairs manager in the Sacramento office of the Los Angeles-based California Immigrant Policy Center, a think tank and advocacy group, said it is premature to pin down the cost of covering the children living in the U.S. and California illegally who are not yet counted by the health care department.

However, “it’s safe to say the state will have to put additional funds into this,” Coleman said.

Figures on those living in the U.S. and California without legal permission are unclear, and the cost of caring for children who already have some kind of coverage is also uncertain, he said.

He noted that not all of the 70,000 or more kids currently getting care through Kaiser Permanente will necessarily go into Medi-Cal. Talks are underway to decide how best to care for them.

In addition, the equation could change if a federal court injunction is lifted on President Barack Obama’s executive order offering citizenship to some of these people, Coleman noted. The Supreme Court in January agreed to hear the case.

If the injunction is lifted, some of the kids in California could be eligible for citizenship under Obama’s order.

Assuming they qualified, they would be covered under regular Medi-Cal, which gets significant funding from the federal government, thus reducing the state’s bill for their care.