The children appeared to be about 10 to 13 years old and one boy, identified as Abu Abdullah al-Britani, is white, BBC reported.
The video showed the children shooting dead five men identified as captured Kurdish fighters who are kneeling.
The child is pictured in full military fatigues and a black hat alongside four other boys, and the prisoners are wearing orange jump suits.
The identity of the boy said to be British could not be verified and the UK Foreign Office has not confirmed this yet.
One of the children, identified as Abu al-Bara al-Tunisi (the Tunisian), addresses the Kurdish fighters in Arabic, promising more violence and saying the battle against them has not yet begun, the BBC reported.
The other children are captioned as Abu Ishaq al-Masri (the Egyptian), Abu Abdullah al-Britani (the Briton), Abu Fu'ad al-Kurdi (the Kurd) and Yusuf al-Uzbaki (the Uzbek).
Children have featured prominently in IS propaganda in which the militant group boasts about preparing the next generation of fighters.
