If ISIS finds they can recruit British children to kill prisoners, or any other terrorist activity, then they'll certainly keep using it. So it makes sense that if it works it Britain, that they'll try it in the US. I'm afraid they will succeed, too.- they only need a few children out of this country of 350 million
One of the hurdles for ISIS in brain-washing children is overcoming a child's innate resistance to violence against others. While children do experiment with hitting and kicking at a young age, they quickly realize that those actions are not acceptable. By using an example of a British child kill prisoners, the propagandists can overcome that initial resistance to hurting someone else. They can point to it as proof that children are capable of violence, and that it is necessary to use them in that way. It might seem abhorrent, but if they believe their enemies are capable of such evil, it makes it feel like less of a problem.
The recent story regarding the four year old child who used an explosive device to blow up a bus full of prisoners is a reflection of the possible avenues ISIS will take to sway anyone "trainable" to follow their ways. This recent action, hopefully, will bring light to the focus on the children that may or may not be a part of the ISIS following and allow our governments to closely track all related to the ISIS followers and their regime.
Based on the above question there really is no evidence that if a British child has been brainwashed to kill a prisoner, that it will anymore negatively effect more children or not. With that said there is also no evidence that their are any brainwashed children working for ISIS. But I do think it is safe to say that if the above is the case, children being brainwashed to any degree is a very serious international issue to say the least.