Turkey has repeatedly denied accusations of its security forces firing on Syrians trying to cross the frontier, which has been closed for several months.
But the Observatory has recorded numerous incidents this year, and said Sunday's toll was among the highest.
The Britain-based Observatory said the group of eight had been displaced by fighting around the northern Syrian town of Manbij, held by ISIL Takfiri group.
They fled northwest towards Idlib province and the border, but were killed "when Turkish border guards opened fire on them as they tried to cross into Turkey," said the Observatory.
It said that four of those killed were children, adding that eight people were wounded, some of them critically.
Since the beginning of the year, at least 60 people -- all civilians -- have been killed in fire by Turkish border guards, the Observatory said.
In May, Human Rights Watch accused Turkish border guards of shooting and beating Syrian asylum seekers.
Turkish President Recep Tayyip Erdogan denied the reports, and the Turkish army says it only fires at armed smugglers, not at civilians.