The judge said he would sentence Sachs to 14 years in prison on Sept. 8.
Police said Sachs had been stalking potential young victims at shopping malls for weeks before the stabbing last October.
He was arrested moments after plunging the 4-inch blade of a hunting knife five times into the boy's back at a Dave & Buster's restaurant in Westbury on Long Island.
Police say he was carrying a typed note indicating he intended to kill a child. They say the victim apparently was chosen at random.