The Yankees released a statement (h/t Sports Illustrated's Kenny Ducey) saying the child received first aid at Yankee Stadium and was "receiving medical attention at an area hospital."
The incident occurred in the bottom of the fifth inning when Yankees third baseman Todd Frazier lined a ball into the seats above the dugout down the third base line.
Per Dan Martin of the New York Post, play was stopped for nearly four minutes as the child received medical attention in the stands before being taken to Columbia Presbyterian Hospital.
Martin also noted Frazier "immediately put his hand to his mouth and went to one knee and other players were visibly shaken by the injury."
The Yankees don't have additional protective netting that extends down the base lines.
In 2015, Major League Baseball issued a memo to all 30 teams that encouraged them to "shield the seats between the near ends of both dugouts and within 70 feet of home plate with protective netting or other safety materials of their choice."