He and the other children copy five older jihadists who execute prisoners in orange jumpsuits and butcher them with knives.
The age of criminal responsibility in Britain is 10 so if the boy was captured and sent back to Britain to face justice he could be prosecuted for murder.
Dozens of British jihadists are believed to be fighting for ISIS in Syria and Iraq under the nom de guerre al-Britani, which simply means British in Arabic.
Last year Assad Uzzaman, 25, from Portsmouth, was killed in Syria while using the name al-Britani.
ISIS has been increasingly using child soldiers - known as the 'Cubs of the Caliphate' amid speculation it is running low on manpower as it retreats from positions in Syria and Iraq and falls back on its stronghold, around Raqqa.
The Quilliam Foundation reported this year that around 50 British children are 'growing up on jihad' and being groomed to become ISIS fighters.