ball girl

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A woman attendant on a tennis court who collects the ball when it is out of play.
• noun A girl who is in charge of the extra balls used in practice or those that are out of play, especially in baseball.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun sports Someone responsible for getting the balls from off the area of play to the players.