One thing to say, and its probably not even worth mentioning but a reminder never hurts. Dont single out your son on the field. Ever. My dad was a big sports nut who could never sit on the sidelines and watch "inferior" coaches. So he volunteered his time. And to be honest, he knew his stuff. He was a good athlete when he was in school and he knew how to instruct others. Baseball, basketball, football, wrestling ......
The thing is, when it was ME on the field, it got personal. He saw every failure on my part as a personal embarassment and didnt mind letting me know it. In front of everyone. He was a slavedriver when it came to practice at home, far past the point where it stopped being fun. I never was great at sports. I guess I should jump on the bandwagon and blame my father for all my failures in life and in sports, and say he drove me away from it. Truth is I sucked at it anyway. What I blame him for is taking the fun out of it, so I didnt even have that.
Anyway, just remember that its a game. Do your best to teach them the fundamentals and teach them teamwork. Dont spare them criticism, but serve it with a smile so it takes the sting out. Be a coach. Plenty of time to be Dad at home and off the field.