BALAKLIYA, Ukraine — The children left this town in August for a free summer camp sponsored by the Russian occupiers, enticed by assurances of gifts and of safety from constant shelling.

“The Russians promised it would be two or three weeks, and then the children would be back,” Nadia Borysenko, 29, told me. Her 12-year-old daughter, Daria, was among 25 children from this town in northeastern Ukraine who boarded a bus to the camp.

Russia did not return them, however. Daria and other children are now across the border in Russia, and Moscow is making it very difficult for families to recover their children.

The youngsters here are among many thousands of Ukrainian children whom Russia has taken from Ukraine and in some cases put up for adoption.

The Ukrainian government count is 11,461 children known by name and taken without families to Russia or Russian-controlled areas. President Volodymyr Zelenskyy told the Group of 20 summit that there are “tens of thousands” more who are known about only indirectly or with less detail.

“Among them are many whose parents were killed by Russian strikes, and now they are being held in the state that murdered them,” he said.

The transfer of thousands of children is a stark reminder that this is not a typical armed conflict. These may be war crimes. They should be a wake-up call to Americans and Europeans fatigued by support for Ukraine.

Do you really want to boost a state sponsor of child trafficking?

Russia doesn’t hide the transfer of Ukrainian children but trumpets it on its television propaganda programs, portraying itself as the savior of abandoned children and showing Russians handing teddy bears to Ukrainian boys and girls.

Russia’s commissioner for children’s rights, Maria Lvova-Belova, boasted last month that she had adopted a Ukrainian boy, and many of these stolen children seem to have been adopted into Russian families.

That is not charity; it may be genocide. A 1948 international treaty specifies that “forcibly transferring children,” when intended to destroy a nationality, constitutes genocide.

Yet the situation is also nuanced. I reached Daria on her cellphone, and she didn’t sound like a traditional prisoner: She has friends, takes classes and can use her phone each evening to call her mom. But she unmistakably wants to go home to Ukraine.

“I miss home all the time,” she said.

Russian authorities allow parents to pick up their kids, but only by traveling to Russia through Poland and then other countries. That means that parents have to scramble to obtain passports and other documents — even as their homes and possessions may have been destroyed by Russian shells — and then take on a substantial expense just as the war has impoverished them. Some parents have managed to do this; most haven’t.

“Of course it’s a war crime when they take our children,” said Dementiev Mykola, a local prosecutor. “And they commit a crime by not making it easy for those children to come back.”

Mykola noted that the summer camp was attractive because it seemed the only way to keep kids safe from Russian shelling. He added that if the Russians wanted to, they could establish humanitarian corridors to repatriate children.

Another mother in Balakliya, Nadia Borysenko’s sister-in-law, Viktoria Borysenko, whose 12-year-old son, Bohdan, is at the camp, said he told her in phone calls that he and others are treated well but want to return. “They are crying and want to come home,” she said.

My best guess is that Russia takes the children to serve as props in its television propaganda shows. And afterward it doesn’t bother to return the props.

Many of the children taken to Russia were removed from institutions such as children’s homes, boarding schools and hospitals. Some of these youngsters didn’t have parents, but when they did, families were apparently not consulted.

Olena Matvienko told me that her 10-year-old grandson, Illya Matvienko, was in the Ukrainian city of Mariupol with his mother, Natalya, when both were badly injured by shrapnel. She died in front of Illya, and Russian troops took the boy not to a local hospital but to one in an enclave that Russian-backed separatists have declared the Donetsk People’s Republic.

The family had no idea what had happened to mother and son until a relative in Russia chanced to see a report on Russian television about heroic doctors in Donetsk saving Illya.

“He was kidnapped,” Matvienko told me. “He was taken forcibly.” She said that Russian authorities prepared papers so that Illya could be adopted in Russia.

To recover her grandson, Matvienko traveled through Poland and Turkey to Russia.

“It was just an accident that this video was seen and reached our family,” she said. “He would have been a Russian boy, and he would have grown up in another family.”

Children are not spoils of war. A government should not traffic in thousands of children. These elementary propositions underscore the moral stakes of the war in Ukraine, and it’s important for the world to stand firmly on the side of right — and to bring Daria home to her mom.

Nicholas Kristof is a New York Times columnist.