Margo Solomon has health insurance for herself and her four children.

But actually getting treatment is another matter.

Ms. Solomon, a 35-year-old mother from the Bronx, says she has struggled to find a doctor who accepts her insurance. And with three of her children coping with asthma, and one with more complicated medical problems, locating a specialist is even more challenging. And once in the door, she cannot afford the costs, including for deductibles and medications.

“I feel like I am all alone out here,” Ms. Solomon said.

She is not alone.

A new study to be released on Monday by the Children’s Health Fund, a nonprofit based in New York City that expands access to health care for disadvantaged children, found that one in four children in the United States did not have access to essential health care, though a record number of young people now have health insurance.

The report found that 20.3 million people in the nation under the age of 18 lack “access to care that meets modern pediatric standards.”