At dawn on May 8, 2023, a 17-year-old Russian teenager named Pavel Solovyov climbed through a hole in the fence of an aircraft plant in Novosibirsk, Russia. He and two friends were looking for a warplane that could be set on fire. An anonymous Telegram account had promised them one million rubles, around $12,500, to do so — a surreal amount of money for the boys.

But when the boys saw the Su-24 supersonic bomber, they got scared. This heavy war plane, versions of which have been pounding Ukraine for the past three and a half years, looked too impressive and dangerous to simply incinerate. After some deliberation, the kids decided to singe the grass around the jet but film it to make it look like the plane was engulfed in flames. The stranger from Telegram had promised to pay only after receiving video evidence of the arson.

Mr. Solovyov is now serving almost eight years in a penal colony. He and his friends, detained within a week, were found guilty of carrying out deliberate acts of sabotage. The children did not suspect that this was, as Russian investigators concluded, a covert attack on behalf of Ukraine. Mr. Solovyov and his friends, according to his mother, had simply been asked to “help the aircraft plant get insurance” for the burned plane. Her son once dreamed of opening his own car repair shop. “Now,” she told me, “all his plans have crumbled.”

This is far from an isolated incident. Small-scale attacks like it are part of a new kind of hybrid warfare being carried out by Russia and Ukraine. Over the years since the Russian invasion, the security services of both countries have discovered a cheap and accessible asset — youngsters who can be recruited for one-off covert attacks, often without even knowing who they are working for. It’s a shocking development in this brutal war: the weaponizing of children.

Stories about cross-border surveillance and sabotage have been circulating for a couple of years. But the phenomenon, as stalemate deepens and both countries look for new ways to strike inside enemy territory, has clearly picked up. To learn more about it, I read through the message histories of recruited children with their handlers, spoke with handlers themselves and even listened to a recording of one of them providing a recruit with a recipe for explosives. Over months, I reviewed hundreds of cases in both countries. It was a crash course in deception and disaster.

This is how it works. First, an anonymous user contacts kids over Telegram, WhatsApp or a video game chat with an offer of a quick buck. Once contact is made, handlers provide instructions. Sometimes these directives are disguised as a “geolocation game.” “Yes, we pay for photos here!” says one online ad posted by recruiters, asking for location-stamped pictures of police cars and ambulances. “It’s like Pokemon Go, but for money.”