The UN's children's agency, Unicef, has said 4.5 million children in South Sudan "are in desperate need of humanitarian support".
The agency attributed the humanitarian crisis to conflict and climate change effects like flooding and drought.
It said child mortality rates and malnutrition remained high in the country.
As many as 2.8 million children are out of school because of lack of access to education.
"Hopes that independence would bring a new dawn for the country’s children have faded," the agency said in a statement.
South Sudan will celebrate its 10th anniversary on Friday.
The country is in the process of implementing a peace agreement that ended years of conflict.
Unicef executive director Henrietta Fore said: “The childhood of many 10-year-old children in South Sudan today has been beset by violence, crises and rights abuses.”
Millions of adults are also in need of humanitarian support in the country.