The study of behavioral practices of children in borderlands intersects two pivotal historical-anthropological research domains: childhood history and border studies. This article delves into unattended border crossings by children in the Karelian region of the Russian-Finnish borderlands during the 1920s. We present the results from an analysis of the two source sets housed in the Republic of Karelia National Archives. These sources comprise formal documents: questionnaire forms completed by teenagers upon their return from Finland and records from interrogations conducted by children’s social security bodies and national security agencies. We identified the factors and circumstances surrounding the illegal movements of underage citizens across the Russian-Finnish borderlands. Our findings reveal that the majority of these young border trespassers were ethnic Karelian teenage boys from borderland villages. The reasons and motivations for crossing the border from Russia to Finland, and subsequently returning home, evolved over time. While personal survival was the primary motivation during the civil war, in more peaceful times, the prevalent drive to go to Finland was economic – to earn money to improve living conditions for both the individual and their parents, reflecting a family survival strategy. We also detail how authorities addressed these transgressions. We conclude that the behavioral practices of these children, in violating national borders, depict them as autonomous actors on one side, yet also highlight their profound dependence on the immediate environment, shaped by its unique political, social, and geographical nuances, on the other.