The children are demanding basic rights, like a better education system, better health care, shelter, jobs for their mums and dads and food.
They say their government doesn't properly look after the 37 million people who are living in poverty in the country.
The 12-day trek - called March for Life - started in the countryside town of Port Iguazu and finally finished in the country's capital Buenos Aires.
Determined teens from all around the country took turns on foot, in buses and caravans.