The boy and the older fighter in the ISIS video are French citizens, a French official told the Associated Press.
The video showed the older fighter speaking in French.
ISIS posted a video Tuesday that allegedly shows the execution of an Israeli-Arab hostage the terror group had previously accused of being a spy.
In the video, a boy appears to kill Muhammad Said Ismail Musallam, 19, by shooting him in the head.
The terror group had accused Musallam of having signed up as a jihadi to spy for Mossad, the national intelligence agency of Israel.
His father told the Associated Press on Wednesday that his son was not an Israeli spy but was lured by ISIS to join them and then killed when he tried to return home after hearing about his sick mother.
Said Musalaam said ISIS tricked his son into joining them with empty promises of women, money and cars.
"They did not want to let him leave because if he comes back, he might be caught by the Israelis and tell them what he had seen. So they wanted to get rid of him," the 19-year-old's father told the AP. "I know my son. I raised him well. I am sure he's not working for the Mossad."
Israeli officials have also denied that he was a spy, the BBC reported.
The ISIS video, which lasts about 13 minutes, begins with Musallam sitting in a room wearing an orange jumpsuit.
Musallam talks about how he had been recruited and trained by the Israeli intelligence service. He also says his father and oldest brother had encouraged him to join.
Musallam is then escorted into a field by the young boy and read a verdict in French by an older fighter who has a long beard.
The footage contrasts from previous videos released by the terror group, which have been mostly in Arabic or English and have included masked militants.
The boy, who is wearing an ISIS uniform, then stands face-to-face with Musallam and appears to shoot him in the head.
After Musallam falls to the ground, the boy shoots him three more times and chants "Allahu Akbar!" (God is Greatest).
Israeli security officials told Reuters that they were aware of the video but could not independently verify its authenticity.
Musallam's family said he went missing while traveling as a tourist in Turkey, but an Israeli official said he went to Syria to fight for ISIS in October.
The ISIS video is the latest in a series of videos that show the killing of hostages. Last month, an ISIS video posted online appeared to show the execution of 21 Kurdish fighters.