They became the latest casualties of a long war that is leaving children dead or wounded in record numbers.
On Monday, the United Nations said that the violence across Afghanistan in the first four months of the year had killed 283 children and wounded another 704, a 21 percent increase over the same period last year.
“The children were playing outside their home and found the mortar round, likely fired during fighting earlier in the day,” Tadamichi Yamamoto, the special representative for Afghanistan appointed by the secretary-general of the United Nations, said in a statement. “A family destroyed in seconds — this horror is just one of too many incidents documented at the onset of this fighting season.”
The “persistent use of indirect and/or explosive weapons in civilian-populated areas,” as well as the use of “indiscriminate improvised explosive devices,” are the biggest cause of casualties among children, the statement said. Deaths or injuries caused by ordnance that does not explode immediately but blows up later are also on the rise.
The violence in Afghanistan is expected to intensify this year, in the absence of any peace effort or any regional consensus on ways to ease the conflict.
The Taliban, who have gained large areas of territory in the last few years, overran the districts of Qala-i-Zal in Kunduz Province in northern Afghanistan and Zebak in Badakhshan Province in northeastern Afghanistan since beginning an offensive late in April. Zebak has been recaptured by Afghan special forces, but Qala-i-Zal, as well as most of the territory in Kunduz, remain in Taliban control, according to provincial officials.
President Trump, in a broad policy review of the war in Afghanistan, is considering a recommendation by his advisers to send several thousand additional troops there. They would be added to the 13,000 international troops in the country now, about 8,400 of them American, that are assisting the Afghan armed forces.
Laghman Province, where the children were killed, has seen increased Taliban activity in recent months, with fighting reported close to the provincial capital, Mehtarlam.
Nazar Gul, 33, who lost his 7-year-old son, Zaher, said the fighting near their village, Omarzayo, had continued over the last 20 days. They had to rush the burial of their relative, the police officer, while bullets were being fired.
In the explosion, Mr. Gul said, Zaher was thrown yards from where the blast occurred.
“Before he died, Zaher came home to ask for money — 10 afghanis — for an ice cream,” Mr. Gul said, the equivalent of 15 cents. “His mother said she didn’t have money. But she gave him cold water, and he went back to play.”
