Another 13 civilians were wounded in the strikes on Khan Sheikhun in Idlib province, the Syrian Observatory for Human Rights said.
Observatory head Rami Abdel Rahman said they were the first deaths in air strikes on an area not held by the Islamic State jihadist group since the truce came into force on Monday.
The Britain-based monitoring group could not identify which war planes carried out the raids, although the government and its Russian ally have regularly bombarded Idlib province.
Under the truce deal negotiated by Moscow and Washington, which came into force on Monday evening, fighting is to halt across the country except in areas where jihadists are present.
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Observers have noted that the deal will be particularly difficult to implement in areas where Fateh al-Sham has formed strong alliances with local rebels, like in Idlib province.
A video posted online by activists in Khan Sheikhun showed two columns of white smoke emerging from a neighbourhood of concrete buildings.
The new truce deal is the latest effort to put an end to the unrest, which began with anti-government protests but has since evolved into a multi-front war that has drawn in world powers.