If that prior behaviour involved violence and injury to others - as does happen, sadly - tasering might be the only way to control the situation. Bear in mind, as well, that the original story comes from the land that worships guns, I'm sure folk can think back to news reports of young children discharging parental guns when they have been left unattended and unsecured.
Hope, you are absolutely unbelieveable!
A 70lb 8 year old with a paring knife (normally about 4 inches long including handle)! Just what danger would you evisage this scenario would present to fully grown adult policemen in full rig-out?
I can envisage no circumstances that justify their actions.
Unless, of course, the child was a martial arts prodigy!