The strike, which hit the town of Khan Sheikun, in the south of the province, appeared to have been carried out by Syrian government ally Russia, the Syrian Observatory for Human Rights monitor said.
The Britain-based group, which relies on a network of sources inside Syria for its information, says it determines what planes carried out raids according to their type, location, flight patterns and the munitions involved.
In Moscow, the defence ministry denied that Russian aircraft had been in the area today, and accused the Observatory of "information provocations".
"The strike hit a street where children were playing. Three of the dead children were from one family and were visiting their grandfather," Observatory director Rami Abdel Rahman said.
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He said the dead children were four girls and three boys, but did not immediately have their ages.
Also in Idlib today, 11 civilians including three children were killed in air raids and regime bombardment of a village in the southwest of the province, the Observatory said.
Idlib province is mostly controlled by a rebel alliance known as the Army of Conquest, which groups Islamist factions with jihadists of the Fateh al-Sham Front, formerly Al-Qaeda's Syrian affiliate.
Elsewhere, Syrian state media said government forces had advanced southwest of divided second city Aleppo, seizing the 1070 district from rebel forces.
Abdel Rahman also reported the advance, saying it would allow government forces to protect areas already under their control on the southern outskirts of Aleppo.
Rebels have sought to penetrate regime territory on the southern outskirts of Aleppo to end the government siege of the opposition-held east of the city.
The Observatory said that regime forces today retook Minyan district in the west from rebels.
No aid has reached rebel districts, where more then 250,000 people live, since July, and rebels have tried several times to break through government lines to relieve the crippling blockade.
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