David's job

George Knox put him in charge of making sure the boys were at all the home games. My guess after that is he was their chaperone and got them food and drinks

"What are we supposed to use? Harsh language?"

I'm assuming initially he was hired for an assistant job or something along those lines. I'm not sure what exactly that would consist of in baseball for a coach, but Knox tells him right in the beginning to "make a note" about firing him on Friday, much like I assume an assistant would do - take notes and run errands. Obviously, later on that consists of being in charge of the boys. Whatever the job initially was, I don't think he had to know about baseball. David really didn't seem to like it much until later on in the movie.