About a million children, double the previous estimate, fall ill with tuberculosis every year, said a study on Monday that also gave the first tally of drug-resistant TB among the young.
"Many cases of tuberculosis and multi-drug resistant tuberculosis disease are not being detected in children," it said.
The team's computer model, based on population data and previous studies, suggests 999,800 people aged under 15 fell sick with TB in 2010.
Around 40 percent of the cases were in Southeast Asia and 28 percent in Africa.
"Our estimate of the total number of new cases of childhood TB is twice that estimated by the WHO (World Health Organisation) in 2011, and three times the number of child TB cases notified globally each year," said Ted Cohen from the Harvard School of Public Health.
The research, published in The Lancet, coincides with World TB Day, which places the spotlight on a disease that claims some 1.3 million lives each year.
The team estimated that nearly 32,000 children in 2010 had multi-drug resistant TB (MDR-TB), meaning the strain was impervious to frontline drugs isoniazid and rifampin and was thus harder and costlier to treat.
This is the first estimate of MDR-TB among children under 15, who constitute a quarter of the global population.
Children are at a higher risk of disease and death from MDR-TB, but react well to medication. They are harder to diagnose, partly because smaller children cannot cough up sputum samples needed for laboratory tests.
Reliable estimates are necessary for health authorities to assign resources for diagnosing and treating the infectious lung disease.
Commenting on the study, Ben Marais of the Marie Bashir Institute for Infectious Diseases and Biosecurity in Sydney, Australia, described it as the "most rigorous effort to date" to assess TB and MDR-TB incidence in children.
"Every effort should be made to reduce the massive case-detection gap and address the vast unmet need for diagnosis and treatment," he said.
The WHO says about 450,000 people developed MDR-TB in 2012 and 170,000 died from it.
Less than 20 percent of MDR patients received appropriate treatment, which promotes further spread of the disease.
Nearly 10 percent of MDR cases are thought to be of the even deadlier XDR (extensively drug resistant) variety which does not respond to a yet wider range of drugs.