The Syrian Observatory for Human Rights said it had documented the deaths of 52 child soldiers, all under the age of 16, who had been part of ISIS's "Cubs of the Caliphate" program.
The programme provides intense military and extremist training to children throughout ISIS's areas of control in Syria, the Britain-based Observatory said.
As many as 31 were killed in July alone, in explosions, clashes, and air strikes by Syria's Air Force and the US-led coalition.
The child soldiers are used to man checkpoints or gather intelligence from areas outside ISIS control, but ISIS has been increasingly using them to execute prisoners or conduct suicide attacks.
So far this year, ISIS has used 18 children as suicide bombers, most recently in its fight against Kurdish militia in northeastern Syria.
"This shows that Daesh is exploiting the suffering of the Syrian people," said Observatory head Rami Abdel Rahman, using the Arabic acronym for ISIS.
"When a child reaches the point of becoming a suicide bomber, this means that he's been completely brainwashed," Abdel Rahman told AFP.
The Observatory said it had received information on dozens more children killed, but that it could not confirm their deaths.
Since the beginning of 2015, ISIS has recruited more than 1,100 children.