The UN's Special Representative for Children and Armed Conflict said children were being tortured in detention and slaughtered in massacres.
Separately the US has accused Syria of planning another massacre, while the UN chief condemned the rising violence.
The UN special representative, Radhika Coomaraswamy, told the BBC her team had returned from Syria with "horrific" reports.
She said she had never seen a similar situation where children were not spared - and even targeted - in a conflict.
"Many former soldiers spoke about shooting into civilian areas, seeing children, young children being killed and maimed," she said.
"We also had testimonies and saw children who had been tortured, and who carried the torture marks with them. We also heard of children being used - this was recounted to us by some children - of being put on tanks and being used as human shields so that the tanks would not be fired upon."
However, she also criticised the opposition Free Syrian Army for endangering children.
"For the first time we heard of children being recruited by the Free Syrian Army mainly in medical and service orientated jobs but still on the front line," she said.
Ms Coomaraswamy said the suffering inflicted on children in Syria was unusual even for combat situations.
"We are really quite shocked. Killing and maiming of children in cross-fire is something we come across in many conflicts but this torture of children in detention, children as young as 10, is something quite extraordinary, which we don't really see in other places."
Image said to show shelling in Homs, Syria, 11 June The UN is pushing for the evacuation of people trapped by shelling in Homs
She said that in recent massacres children under the age of 10 had been summarily killed.
The UN's annual report on children and armed conflict cited one attack on the village of Ayn l'Arouz in Idlib province on 9 March.
It quoted a witness saying how several young children were forcibly taken from their homes and "used by soldiers and militia members as human shields, placing them in front of the windows of buses carrying military personnel into the raid on the village".
Other children described being beaten, blindfolded, subjected to stress positions, whipped with heavy electrical cables, scarred by cigarette burns and in one case subjected to electrical shock during interrogations, the report said.
A team of UN military observers is currently inside Syria as part of a six-point peace plan brokered by UN-Arab League envoy Kofi Annan.
Despite all international diplomatic efforts, bloodshed is continuing on a daily basis. Activists said more than 100 people were killed on Monday across the country, with the heaviest casualties in northern Idlib province.
Activists say the regime of President Bashar al-Assad is deliberately targeting civilian populations, while the government blames the violence on "armed gangs".
US state department spokeswoman Victoria Nuland said there were fears the Syrian government "may be organising another massacre", in the town of Haffa in Latakia province, where UN military observers have been denied access.
"We are calling this out now in the hope that we can stop what could be a potential massacre," she said, adding: "People will be held accountable."
Earlier this month, activists said Syrian government forces killed 108 people in the region of Houla, in Homs province, and 78 people in the village of Qubair, in Hama province.
He says the UN observers are eager to get to the area to prevent any massacre happening, but the government forces are apparently blocking them.
Rebel fighters in the town are besieged by the government troops, who have been using artillery, tanks and attack helicopters.
There are increasing fears among UN observers about the use of attack helicopters.
In a statement, UN Secretary General Ban Ki-moon expressed "deep concern at the dangerous intensification of armed violence across Syria over the past several days".
He said: "The Syrian government's intensive military operations, including the shelling of Homs and reportedly other population centres, as well as firing from helicopters on Talbiseh and Rastan, are resulting in heavy civilian casualties and human rights violations."
Our correspondent says the situation in Homs is of particular concern, with many families trapped by shelling. The UN wants to evacuate people from the battle zone but he says that, so far, this has proven impossible.
There are also reports that the army has shelled a night-time protest in the eastern city of Deir al-Zour, with activists saying 11 people have been killed.
Meanwhile, British Foreign Secretary William Hague, who is visiting Pakistan, said foreign military intervention was not being considered for Syria and that all efforts were being made for a peaceful transition.
"I think we should not think about it in terms of another Libya," he said!!.