It seems that the person making these accusations, Murat Kurnaz, is a former Gitmo detainee. Yes, it seems he was innocent and should have not been detained. But this fact is independent of the accusations he is making. Here are his words from the video:

"The worst thing that I saw...I saw children over there...9 years old, 12 years old...and many kids who were 14. They got tortured in the same way...they got beaten up and they had to live in small cages for many years."

Anyone can see that he has a major grudge with the US government, and that is understandable. It is obvious that this is his attempt to vilify US Military, coming from someone with an agenda.

And why is this the first time he has ever mentioned children being held at Gitmo? Once again, the allegations seem to coincide with his book release. Simple propaganda IMO. If you read about him you will notice that there is NO mention of children detainees, or reference to children being tortured at any time prior to these allegations.

Murat Kurnaz
MotherJones Blog: Murat Kurnaz Details His Plight Before House Subcommittee
Murat Kurnaz has been released from Guantánamo! | Amnesty International

Here is a 60 Minutes interview with him, and NOWHERE is his interview does he mention children being held captive at Gitmo.

Murat Kurnaz - Mahalo

In the OP's video link he stated that that the children being beaten was "The worst thing that he saw". If so, why never mention it until now? I empathize with what he went through at Gitmo, but it is no reason to play the "child card" in an attempt to promote his book.

Russia's ambassador stated the following:

"900 youngsters have been held in coalition prisons in Iraq."

First off, where does he get these numbers?? From a former detainee that is promoting his book? If not from him, then from where. Sounds like propaganda to me.