The children were peacefully playing during their first break of the day when government forces launched mortar rounds at their nursery.
Heart-wrenching photos from the scene show young children covered in blood in the rebel-held city of Harasta, a rural suburb of the country's capital, Damascus.
One boy winces in agony as a doctor checks his heart. Another injured girl looks vacantly into the camera after suffering an assault too horrific for words.
The toys the children were playing with are left strewn across the tiled floor which is splattered with blood.
The Britain-based Syrian Observatory for Human Rights monitor said 17 people, most of them children, were injured in the shelling.
Local reports claim forces loyal to the government fired artillery that hit the kindergarten at around 10am local time.
Volunteer rescue group, the Syrian Civil Defence force also known as The White Helmets, attended the scene.
One of the children killed has been named by the group as Jihad Qadado, son of martyred Syrian Civil Defence force member Abdulfatah Qadado.
An AFP photographer saw the body of one child, a girl, lying on a bed at a makeshift hospital, her face bloodied and her clothes torn.
'Nothing can be done. Nobody can stop the planes,' said Bebars Mishal, an official with the 'white helmets' in eastern Aleppo, which digs victims out of the rubble and runs an ambulance service.
He said there was no way for rescue workers or medical staff to prepare in advance of the expected resumption of attacks: 'All we can do is take precautions and be ready 24 hours a day.'
The government and its Russian allies say they target only militants, and that fighters are to blame for civilian casualties by operating in civilian areas.
Western countries say the bombing has deliberately targeted hospitals, aid workers and bakeries and Washington has accused Moscow of 'war crimes'.
Rebels say the aim is to drive out civilians, some 275,000 of whom remain in the besieged zone.
'They call it a ceasefire. The regime hasn't let us hear the end of it,' said Modar Shekho, a nurse in rebel-held eastern Aleppo. 'As usual, when it ends they will let the bombardment loose. We've gotten used to this.'
More than 11 million people, nearly half Syria's population, have been driven from their homes by the war since 2011, including 5 million who fled abroad as refugees.
