Learning to slow the game down begins at practice! When you're in the middle of your drills really focus in on what you're working on. For example slowing the game down for a 10 and under player may be just focusing in on fielding the ball correctly, working on staying down on grounders or taking the correct path to the ball and catching it, while this may sound simple to an advanced player, these basic fundamentals begin to build the confidence in the young player allowing them to understand that they can repeat this in game situations. For an 14 under infielder it may be working on first and third situations and understanding how to properly execute the play when the game is on the line.
For the 16 and 18 unders fastpitch softball player it may be the understanding how to catch up to the velocity of the pitching at the level, spending time in the cage, on the tee and off of live pitching developing a repeatable swing as well as learning how to pick up rotation of the thrown pitch.
As you can see the common denominator here is no matter what level of softball you play or coach, if you want to slow the game down building the confidence and trust in the players ability is critical. Practice is where this is built, as a player you need to focus in on every drill, as a coach you need to place them in game situations that gradually push them out of their comfort zones always challenging them to the point of success.
* Point to coaches, challenge to the of the players ability, expecting a new player to do something that's beyond their means will only speed the game up for them and defeat the entire process.