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.”

While Medicaid and many private insurance plans recommend or require that all of those services be provided, under the umbrella of what is known as the medical home, the study found that millions of insured children are not receiving many of the benefits.

There are many children with insurance who cannot get primary care and those who do can often have problems getting specialty care.

As President-elect Donald J. Trump, a Republican, vows to repeal some, if not all, of the Affordable Care Act, which extended health care coverage to an additional 20 million people, the report’s authors worry that even more children could have trouble receiving the care they need.

“The fact that more than 20 million children in the U.S. experience insurance and noninsurance barriers to getting comprehensive and timely health care is a challenge that needs to get the highest-priority attention from the new administration,” said the report’s lead author, Dr. Irwin Redlener, president of the nonprofit Children’s Health Fund and a professor of pediatrics and health policy and management at Columbia University.

Over the past two decades, the number of children without health insurance has steadily decreased to 3.3 million last year from around 10 million in 1997, according to an analysis of federal data and the federal government’s 2015 National Health Interview Survey.

The effort to extend coverage began 50 years ago with the creation of Medicaid, which provides health insurance for the poor. It continued more recently with the Children’s Health Insurance Program, which offers low-cost coverage to those who make too much money to qualify for Medicaid and, under the Obama administration, with the Affordable Care Act, offering subsidized coverage and state exchanges.