“The French air strike (on Sunday) on an IS training camp in eastern Syria killed at least 30 Isis fighters including 12 from the 'Cubs of the Caliphate,'” said Rami Abdel Rahman of the Syrian Observatory for Human Rights.
Isis calls its child soldiers “Cubs of the Caliphate”.
Abdel Rahman said foreign Isis fighters were also among the dead, and that he strike had wounded around 20 people.
The raid took place in Syria's eastern province of Deir Ezzor, near the Albu Kamal border crossing used by IS to link the Syrian and Iraqi parts of its so-called “caliphate”.
President Francois Hollande said on Sunday that six French warplanes hit an Isis training camp near Deir Ezzor city, and that more strikes could follow in the coming weeks.
“We struck militarily an extremely sensitive site for (IS),” said French Defence Minister Jean-Yves Le Drian, describing it as a “strategic hub” for militants travelling between Iraq and Syria.
It was France's first air strike in Syria as part of the US-led coalition fighting the extremist group there and in Iraq.
France was already bombing Isis targets in Iraq and had carried out 215 of the nearly 4,500 air strikes there.