Around two minutes after the missile launched, its high-explosive warhead detonated no more than 50 yards from the playground, spewing fragments of hot metal through the air.

Danylo and Alina were cut down, along with seven other children. Among the other victims were a 9-year-old who loved playing with Legos and a 7-year-old who was driving home with his parents.

It was the deadliest single strike on children since Russia’s 2022 invasion. The attack, which killed a total of 20 people, shocked Ukraine and exposed the limits of President Trump’s efforts to end the war.

Russia has intensified strikes across Ukraine since Trump took office pledging to bring peace. On Palm Sunday, two ballistic missiles killed 35 people in the northern city of Sumy in the deadliest strike this year.

“Yes, the war must end. But to end it…we must pressure Russia—the one choosing to kill children instead of choosing a cease-fire,” Ukrainian President Volodymyr Zelensky said in response to the attack on his hometown of Kryviy Rih.

This account of the blast and its aftermath is based on interviews with investigators from Ukraine’s main security and intelligence service, the SBU, and witnesses, as well as a review of footage from surveillance cameras at the restaurant and police body cameras. The Wall Street Journal saw no evidence of any military presence in the area.

Russia said it killed 85 servicemen and foreign officers in a “high-precision strike” on the restaurant, also destroying 20 military vehicles.

After school on April 4, the day of the strike, Danylo took the family’s pet Yorkshire terrier to the groomer and rushed over to Alina’s house. They hadn’t seen each other for days because he had received a bad grade the previous week and had stayed home to focus on his schoolwork.

“He said he really missed her,” said his mother, Natalya Nikitska, who gave him permission to go and see Alina.

Danylo helped Alina’s mother, Marta Kutsenko, carry her groceries up four flights of stairs to their apartment and waited for Alina to get ready. “She was trying on a lot of clothes in her room,” said Kutsenko.

The war had cast a shadow over Danylo and Alina’s early teenage years, but they adapted. Like teenagers anywhere, they had their first romances and went to McDonald’s, though most of their classes took place online because of the threat of Russian attacks.

It was around 5 p.m. when Alina headed out with Danylo and two friends, walking through residential neighborhoods of Kryviy Rih, an industrial town built on an iron-ore seam. Danylo called his mother to ask for permission to stay out later than usual so he could spend more time with his girlfriend.

Their path took them past Soviet-era apartment blocks toward the RoseMarine restaurant, where staff were winding down after a busy day. The restaurant had hosted a birthday party as well as a beauty business forum, organized by a Kryviy Rih business association. Around 80 people, mostly women, attended the event.

When the last guest had left around 6:30 p.m., the kitchen staff went out for a cigarette break behind the restaurant. A cook was testing out a new GoPro camera and was filming when an alert flashed up on the phone of one of the kitchen staff, who said: “There’s a missile coming.”

The next instant, an explosion threw the cook to the ground with the restaurant’s head chef on top of him. The missile had exploded less than 100 yards from the restaurant, hurling fragments that cut through metal, trees and flesh.

Russia often struck again after an initial attack, but the cook’s urge to help was stronger than his instinct to seek shelter. The camera kept filming as the cook sprinted, breathing heavily, toward a woman standing over a small boy in a yellow jacket lying face down on the ground beside a tricycle.

“Call the ambulance, please!” she implored. The cook tried to call emergency services but the network was down.

Police body camera footage captured scenes of anguish at the site of the blast. Survivors pulled the wounded out of cars, including the lifeless body of a 7-year-old who had been driving past with his parents.

On the playground, a body lay next to a merry-go-round. Another child’s body was supine on a bench beside two distraught adults consoling each other. A paramedic tried in vain to resuscitate a 3-year-old.

“They’re all 200s,” said the policeman wearing the body camera, using a military code word for dead.

Danylo’s parents had heard the explosion and could see smoke through the window. It was rising from the direction of Alina’s house. They couldn’t get through to Danylo but at first hoped it was just that the phone network was down. Minutes later, when they still couldn’t reach him, they jumped in their vehicle and joined the crush of ambulances and rescue vehicles heading to the scene.

As Danylo’s parents searched frantically for their son, the air-raid siren sounded again. Russian drones were attacking another residential neighborhood of the city.

A short distance from the playground, several bodies had been covered up. Roman Nikitskiy, Danylo’s father, drew back a towel laid over one of them to reveal the jeans and Nike sneakers his son had been wearing. Danylo’s hand was still clasped together with Alina’s.

“Even the blast couldn’t tear them apart,” said Nikitska, Danylo’s mother.

Marta Kutsenko’s own desperate search for her daughter had also brought her there, uniting the parents in anguish over the loss of their only children. A video showed the three of them bent over the lifeless bodies. “My sunshine, my darling,” Nikitska said. The two friends they had been walking with were also dead.

The children were buried over three days of mourning, Danylo and Alina in matching white coffins.

Days after the attack, the ambassadors of 32 countries including the U.S. gathered at the playground as snow drifted down.

An air-raid siren sounded as Roman Nikitskiy addressed the ambassadors with an appeal to Trump. “You are president of the most powerful country in the world,” he said. “Our kids are asking you to lend us a Patriot so that together we can stop the enemy: Russia,” referring to a U.S. missile-defense system.

Nikitskiy also called on other Western countries to help Ukraine build a bomb shelter next to every playground in the country.

Russia’s representative to the United Nations said during a Security Council session that a Ukrainian air-defense interceptor had caused the civilian casualties. But Ukraine’s SBU security service said fragments of the projectile recovered from the site and seen by the Journal proved Russia was lying.

More than a week on, dazed residents were still picking pieces of shrapnel and glass out of the grass. Natalya Kalynychenko was filming a video to prove to relatives in Russia that the strike hadn’t successfully targeted the restaurant, as Moscow claimed.

“It’s pointless to try to explain anything to them,” said Kalynychenko, whose apartment overlooks the playground. Still, she tried.

Children came to look with curiosity at the swings, seesaw and sandpit covered with flowers, candy and stuffed animals.

“Where was the explosion?” said one boy.

“Let’s go look at that hole in the ground,” said another, pointing at a crater gouged by a fragment of the missile.

A 9-year-old called Kyrylo stood quietly looking at the tributes, before pulling a toy truck from the pocket of his padded jacket. “I’m not a little boy anymore; I don’t need this toy,” he said, adding it to a mountain of stuffed animals. “Does it mean we’ll never be able to play here again?”

Alina and Danylo were buried side by side, near 3-year-old Tymofiy—the youngest victim of the strike. A stuffed beaver that Danylo had given Alina was placed alongside her in the coffin. With the last 1,100 hryvnias ($27) in Danylo’s piggy bank, his parents bought a bouquet for her grave. They were the only flowers that hadn’t faded when they returned to the cemetery a week later, his mother said.

On the ribbon, they wrote: “To my Alina, forever yours—Danylo.”

Write to Isabel Coles at isabel.coles@wsj.com