It is not quite as simple a picture as you paint it.
Twenty years ago, the child would have been physically subdued. Depending on how violently and effectively he fought, physical subdual may have resulted in bruises, or in a broken wrist or dislocated shoulder as the police attempted to remove the weapon from his possession. It is also possible the police would have been injured, perhaps severely; again, depending on how viciously the berserk child fought. 20 years ago, tasers and OC weren't as widely available or widely used by police. There's a REASON why cops use tasers and OC these days: it cuts down on injury to both perps and officers.
A truly berserk 8yo with a stabbing implement and the will to attack all-out is perfectly capable of killing an adult, if the adult doesn't defend himself vigorously. It has happened. That vigorous defense may result in bodily harm to the child that is worse than OC.
There are liability issues involved, as well as department policy issues. It may very well be that the police were FORBIDDEN by dept policy to attempt to physically disarm the child while the child was in possession of a stabbing implement. These days, most dept's frown on physical subdual of armed subjects and mandate the use of OC or Taser.
If that had been MY child... I'd have been glad that the police subdued him without serious harm, and that they stopped him from actually stabbing someone and thereby getting himself in even WORSE trouble.