4 ideas
Emil Tischbein, a widowed hairdresser's son from a small town, travels alone to Berlin carrying 140 marks pinned inside his jacket for his grandmother. A man in a bowler hat who calls himself Grundeis gives him sweets, Emil falls asleep, and wakes to find the money gone, so he follows the man off the train rather than going to the police, fearing arrest for having painted a moustache on a town statue.
The children run the pursuit like a small organization with distinct roles: a strategist who makes plans, runners and shadows who keep eyes on the target, reserves held back at a base, and one boy stationed at home by the telephone as a fixed relay hub. Because every mobile member reports to one stationary point, information stays synchronized and the group can adapt without everyone being in the same place. The key design choice is that the least glamorous role, staying still by the phone, is what makes the whole moving network work.
Instead of arresting the thief, which children cannot do, the gang surrounds him in broad daylight with dozens and then hundreds of children who follow him everywhere. This removes his anonymity, turns public attention against him, and makes escape or quietly spending the money impossible. It works because a crowd of witnesses exerts more pressure than any one pursuer, and it needs no authority, only numbers and visibility.
At the bank, the thief claims the banknotes are his, and it is Emil's word against an adult's. Emil proves ownership because he had pinned the notes into his jacket lining, leaving pinholes that only the true owner could know about. The mechanism is that a small, unplanned physical mark on an object can serve as authentication, beating the credibility advantage of an adult's claim.