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).