No child is safe from the bloody conflict in Syria amid reports of the "shocking" scale of brutality against youngsters.
War Child said young Syrians were being deliberately murdered in execution-style killings, raped, abused, used as human shields and even enlisted against their will to fight.
A report said the Syrian conflict was "disturbingly unique" in its deliberate targeting of children, warning that no child was now safe.
War Child urged the UK Government to step up efforts to protect children caught up in the fighting, saying it believed between 500 and 1,300 children had been killed, while eight-year-olds were being enlisted as soldiers.
Girls and boys aged 12 have been sexually abused, more than 600 children have been placed in detention centres where torture is commonplace and 49 children were massacred in one incident, said the report.
One 10-year-old girl saw her father killed, saying: "I recognised him from his boots. His face was covered in blood. My mum was shaking him."
Rob Williams, chief executive of War Child, which is providing emergency assistance to Syrian children refugees in Lebanon, said: "The Syrian conflict must now rank as one of the worst for the depth and scale of abuses against children.
"When adults go to war they have a legal duty to protect children yet neither side is protecting children in the areas they control.
"In fact, there are deliberate violations of thousands of innocent children including murder, rape and abduction. These actions will scar Syria for generations.
"The UK has consistently failed to achieve any kind of influence to curb these gross abuses. It must now ensure its diplomatic efforts directly target the exploitation and killing of children, and build a child-sensitive dimension into its work in the future.
"With Syrian government-backed forces becoming more desperate by the day and the crisis deepening as it tightens around Damascus, not one of its two million children and young people can now be considered safe. The UK and international community must act to save children's lives."
International Development Secretary Andrew Mitchell said: "Any form of violence directed against children in Syria is shocking and abhorrent. Britain has worked to exert international pressure to bring an end to the violence, and we will continue to work to this end.
"Earlier this month, Britain doubled its humanitarian support to people inside Syria caught up in the violence, and refugees forced to flee to neighbouring countries. We have led the way in ensuring that emergency food supplies, medical support and other life-saving aid is reaching those who need it most, including tens of thousands of families, children and young people."