What is BALL BOY (BALL GIRL OR BALLKID)?
BALL BOY (BALL GIRL OR BALLKID) meaning a person, commonly a child tasked with retrieving tennis balls from the court that have gone out of play and supplying the balls to the players before their service. Ball boys in net positions normally kneel near the net and run across the court to collect the ball, while ball boys in the back positions stand in the back along the perimeter of the arena. Ball boys in the back are responsible for giving the balls to the player serving.
reference: www.playmoderntennis.com: Tennis Terms