By Stephanie Stahl

PHILADELPHIA (CBS) — The coronavirus pandemic in the United States has reached new heights. Coronavirus infections among U.S. children grew 40% in the last half of July. This comes as schools and parents contemplate the safety of sending children back to school.

There was an explosion of COVID-19 cases among children in the last two weeks of July, according to a new report. The report shows that cases may have been slow to start for kids, but not anymore.

As kids enjoy a summer day at Seger Playground on Lombard Street, numbers of COVID-19 cases among children are increasing.

As of the end of July, more than 338,000 children tested positive, according to the Academy of Pediatrics — 97,000 of those cases were reported in just the last two weeks of the month, that’s a 40% jump.

Coronavirus is mild for most children. Many have no symptoms but they put others at risk.

“They can bring virus home to their families, many of whom may have individuals who are at high-risk of having adverse experience with an infection,” said Dr. Susan Coffin from the Children’s Hospital of Philadelphia.

Increasing numbers of children with coronavirus is a concern with some schools opening, like one in Georgia where students were seen crowded into a hallway, many not wearing masks.

That school is now temporarily closed for cleaning after nine students and staff members tested positive for COVID-19.

“They kind of just sent us to the school and used us as guinea pigs to see what would happen later on,” Hannah Watters said.

With the U.S. now passing the 5 million mark of known COVID-19 cases, some health experts say the actual number may be higher.

“It is one of the worst public health tragedies in the history of the United States, certainly in the last 100 years,” said Dr. Peter Hotez.

It’s a big worry for Mendy McNulty, who’s deciding whether her two sons should go back to school.

“I am nervous about sending them into a building with hundreds of other people when we have been isolated for so many months here in our home,” she said.

Research also shows that older children can transmit the virus just as much as adults, and children younger than 5 carry a higher viral load than adults. This is raising even more questions about their role in transmission.

Also, Black and Hispanic children and adults are impacted more severely by COVID-19.

Stephanie Stahl