In the video, which has not yet been independently verified, the child says that the group will kill "kuffar" -- nonbelievers -- "over there," referring to the West, while the adult threatens and insults British Prime Minister David Cameron.
The speaker's accent and dress bring to mind the previous masked face of the terror group, Mohammed Emwazi, otherwise known as "Jihadi John." Emwazi was understood to have been killed in Raqqa late last year in an airstrike that specifically targeted the Briton
. His absence lends credence to Western intelligence agencies' belief that he is indeed dead.
Five men appear at the beginning of the new video, confessing to spying on behalf of the British. Much of the video is taken up with the five speaking, looking directly into the camera. Most of the men say they are from Raqqa, the de facto capital of ISIS' territory in Iraq and Syria. The video was purportedly shot there.
A spokesperson for the British Foreign Office says that it is "aware" of the video and is examining its content.
Following the confessions, the five are then seen kneeling in the desert, each with a masked militant standing behind them. The middle militant then addresses Cameron, speaking in a British accent.
He calls the Prime Minister a "slave of the White House" and an "imbecile."
"Only an imbecile would dare to anger a people who love death the way that you love your life," he says.
"Know that today, your citizenship is under our feet," the man in the latest video continues, addressing British citizens and pointing his weapon at the camera. "And that our country, the Islamic State, is here to stay, and we will continue to wage jihad, break borders and one day, invade your land, where we will rule by the Sharia."
Islamic State is another name for ISIS, and he refers to Sharia law, a strict interpretation of Islamic law.
After his speech, the five captives, dressed in orange jumpsuits similar to those that ISIS has made previous murdered captives wear, are killed with a gunshot to the back of the head.
A spokesman for the Prime Minister told CNN that the video is an ISIS propaganda tool.
"It's desperate stuff from an organization that really does do the most utterly despicable and ghastly acts and people can see that again today," Cameron said on Monday. "This again shows what an appalling organization we're up against."
The 10-and-a-half minute video ends with the child, dressed in military fatigues and wearing an ISIS headband, saying, "we will kill the kuffar over there," in English and pointing into the distance. It features some of the group's motifs, such as the jumpsuits and the use of children to further its propaganda goals, as well as its slick production values.
The video surfaces as the group suffers setbacks, with top leadership killed and in the loss of territory including the city of Ramadi, where Iraqi troops have liberated the city.