He also loved to sing. His favorite song was by Myo Gyi, a Myanmar rock icon, written as a letter from an older brother to a younger brother encouraging him to never lose hope.

On Saturday afternoon in Yangon, as he was playing in the street with friends, security forces shot him in the head, the single bullet piercing his skull and killing him instantly. He was one of more than 100 people killed over the weekend, including increasing numbers of children, by Myanmar security forces, as they crush all opposition to the Feb. 1 military coup. At least seven of the dead were children younger than 16.

“We miss him,” Khin Su Hlaing, his mother, said through tears. “The house is silent without him.”

The Myanmar military’s attacks on children, civilians and peaceful protesters, human rights experts say, constitute acts of terrorism, designed to subjugate a population that has risen up against the army’s seizure of power.

After an especially bloody weekend, protesters and human rights groups are calling for stronger action from the international community, and warn that children are at particular risk.

“These are barbaric criminal acts, calculated to provoke a state of terror in the general public with the purpose of terrorizing the entire population,” said Marzuki Darusman, a member of the Special Advisory Council for Myanmar and the former chair of a U.N. fact-finding mission that investigated the military’s crimes against Rohingya Muslims. “The actions of the Myanmar military are the actions of a terrorist group, under any United Nations definition of the term.”

UNICEF, meanwhile, said the longer-term consequences of the crisis on children could be “catastrophic,” as delivery of critical services for children, including key vaccines and vitamin supplements, has ground to a halt in the impoverished country.

UNICEF’s tallies show that more than 35 children have been killed by the security forces over the two months since the military took power. More than 450 people have died in total.

The Myanmar military, an institution with a long history of human rights abuses and war crimes against civilian populations, took power in a Feb. 1 coup, deposing the civilian government led by Aung San Suu Kyi.

The same day the military was marking Armed Forces Day, Sai Wai Yan was playing with his friends in the Mingalar Taung Nyunt neighborhood, where railway workers and their families live in dormitories provided by the government. Security forces had increased their presence in the area since the workers went on strike over the coup.

A neighbor living close to the 13-year-old’s family said he heard the sound of gunfire and rushed into the street, where he saw Sai Wai Yan. Security forces had entered the neighborhood after they saw some garbage cans blocking the street.

“They came in and asked who did that,” the neighbor said, speaking on the condition of anonymity because he fears for his safety. “They started shooting after that.”

Another, 11-year-old Aye Myat Thu, was killed in Mawlamyine, a trading port in southeast Myanmar. She was buried along with her recent drawing of Hello Kitty and her favorite toys and books.

Soldiers surrounded Sai Wai Yan’s body and dragged it away, multiple witnesses said. His family waited more than a day before they were able to hold a funeral for him, negotiating with officials at a military hospital to claim the corpse.

As the family prepared the body for its final resting place, they tried their best to hide the gunshot wound to his head. They applied thanaka to his face, a distinctive yellow paste that people in Myanmar use to protect their skin, and laid him in a coffin, surrounded by yellow chrysanthemums, red and pink roses, and white jasmine flowers.

“We won’t forget this day until the end of the world,” his mother said, screaming and crying.

On Monday, his family set up a small altar, placing offerings of his favorite food — sugar cane juice and fried rice topped with an egg — in front of his smiling portrait. They sang his favorite song, “Nyi Lay,” meaning “younger brother,” by Myo Gyi. He had been singing it, too, they said, that morning just hours before his death.

“Our lives are very short, just like a flame,” the song says. “We are all going to be ashes.”

Cape Diamond in Yangon contributed to this report.