First of all you can tell the entire team before they are ever replaced anywhere that when we replace you it is not about you unless you have acted out and deserve to be removed from the game for conduct; otherwise, it is never about you. It is what can be best for the team and is done to give other players a chance or turn to in this case to pitch. You can also tell them ahead of time that Little League baseball has a pitch count and this is the number of pitches we want you to throw for this game. If we take you out sooner it is because we feel a change is necessary. Each outing, we hope you can show improvement and keep practicing.

I would first start by letting them know that they have done a fabulous job. In most cases, every player needs a chance to play all positions and you could let the player know that it was simply another players turn!

Walk out to the mound and greet him cheerfully..."hey buddy-boy, how ya doing
out here", etc. Regardless of how he has performed, tell him you are very proud of
him for working so hard. Tell him that we're going to let ______give it a try and
to keep his chin up and be ready to work some more on pitching next practice.
practice.

The kid should know when you are going to pull him before hand. I know some teams dont have the depth but I tell my kids walk three batters in a inning witch mostly happens in that age that if I come out they are done. I usually let them get the last out if no runners and 2 outs but if runner gets to 3d and cant find strike zone just call time and walk to mound and say good job take the ball.

One way to prevent heartache is to tell the pitcher that he is going to face
two batters before the inning starts. Then, if you pull him, it is not big deal. If he is
doing well, let him go. Everyone wins!