There has been an increase in the number of children caught in conflict last year despite the pandemic and the UN's call for a global ceasefire, according to Save the Children.
One in eight of the world's children, more than 300 million, live in conflict zones where they are at risk of becoming child soldiers.
During 2020, some 337 million children were living near armed groups and government forces that recruit children, a report by the charity Save the Children said on Tuesday.
"In the shadow of Covid-19 and the UN's call for a global ceasefire, more children than ever before are caught in the crosshairs of the deadliest war zones," said Inger Ashing, Save the Children International's chief executive.
Children are now "more likely to be injured, recruited or killed," she added.
Nearly 200 million children live in the world's deadliest war zones, up 20% from 2019, according to the report.
It said boosting school access was vital in fighting forced recruitment.
The United Nations called for a global ceasefire last year to help fight Covid-19, but armed groups have continued fighting in countries including Afghanistan, the Democratic Republic of Congo, Nigeria and Yemen.
"War on children"
The exact number of child soldiers is unknown, but over 8,500 children were recruited and used as fighters or in other roles by mostly non-state armed groups in 2020, a 10% increase from the previous year according to UN data.
That number is likely to be only a fraction of actual cases, the charity's report said.
"Millions of children have known nothing but war with appalling consequences for their mental health, ability to go to school, or access to life-saving services. This is a stain on the international community," Ashing added in a statement.
The forced recruitment of children for use in armed conflict is one of the worst forms of child labour, alongside abuses such as trafficking for sexual exploitation, according to the UN International Labour Organisation (ILO).
Girls, who made up 15% of UN-reported cases of recruitment in 2020, often act as spies or suicide bombers and are especially at risk of abuse, according to Save the Children.
The report laid out recommendations for stopping "this war on children" including holding perpetrators of grave violations to account and ensuring access to education to protect children.