The referee is the person who is in control of the game, in charge of enforcing all the rules. In U6 through U10 you'll probably have only a center referee. In the other divisions (U12 up) you should have two more officials (see Law 6).
Too many times referees are treated by spectators and coaches as if they are the enemy. Just recently I was told that one of our best young refeees was so verbally abused that it made her visibly upset. SHAME on the coach, who from my perspective should be sanctioned.
Experienced referees usually know how to handle these type of coaches, but younger referees may not. The worst example a coach can give is to be beligerent and nasty. It gives others a reason to act the same way. I have sternly warned parents on teams I coached when they cheered because an opposing player was ejected or a call went against the other team.
No matter how experienced, or what age the referee is, as a coach you must respect what they do. Sure there are times that a referee misses a call, hopefully not a serious one. I started refereeing decades ago but always tell coaches: "I have yet to referee a perfect game. Let's hope this is the one."
It is extremely embarassing for a coach to complain about a call when the referee was absolutely correct. The problem with that is that there are spectators behind that coach who are convinced that the coach is correct and the referee wrong. Their perspective of that call may never be corrected and they go on forever with an incorrect interpretation.
Part of teaching players fair play is to be an good example.
There was a coach who came to me when I was Director of Coaching, as I was waiting to referee the next game. He started to complain about some referee calls by one of our younger referees. I looked at him, pointed at my referee badge and said: "When you have one of these, I'll talk to you about it." It was well heeded as he decided to become a referee. Since then, he has not complained about referees and he often relates the conversation to others now. That is not to say you shouldn't explain a proper call made by a referee to anyone who asks you (nicely).
So I strongly advise that you learn the Laws of soccer well. Additionally I recommend that you become a referee. It's easier than you think at the lower levels.
Let the kids play the game, you as a coach keep things positive, and the referee do the job without negative comments from you. Set a good example and control your spectators.