In baseball, “on-deck” refers to a player who is next in line to bat.
In a professional game, the batter who is on deck traditionally waits in a location in the foul territory called the on deck circle (there are two circles, each used by a team).
Due to safety concerns, most youth fields have a caged “on deck” circle to protect a player from an errant ball.
If particular field does not have this separate area, check with the league to see if there is a designated warm-up area. You will probably need to assign a coach to monitor in this situation to minimize the risk of injury.
The player next in line to bat behind the on deck batter is referred to as being in-the-hole (note: in youth baseball, an in-the-hole player is responsible for retrieving a bat used by a previous batter).
Good explanation about the on-deck circles. Please note that the circles identified in the second image are NOT on deck circles for MLB games. Those are the places where coaches stand during pregame warmups to hit fungoes to the team and/or used as on-deck circles during batting practice when the cage is out. They would never be used during a game, the actual on-deck circles during the game are near the dugout.
Those circles are not typical on diamonds these days, they have been removed in favor of temporary circles that are added for warmups and removed before the game starts.