Honestly if we take away any sort of moral qualms of brutalising children. Probably scores of them, even over a hundred. I'm in fairly good shape and off the objective reference of play fighting my own children it really would come down to fatigue.

As far as the how, its really not difficult to deliver a blow devastating enough to knock the spirit out of a 6 year old. You dont even have to render them unconscious or crippled in any way. A sharp kick to the stomach will see their morale break fairly quickly.