I agree with It Is....I address the captains usually with humor during the pre-game expectations that are given. I always tell them to do their best, play till they hear the whistle, play with good sportsmanship, etc...but there are a few things that I don't tolerate, and I explain those. Dissent, taunting and anything that borderlines on racist comments or actions. I jokingly ask if they have ever argued with a referee and won that argument. Most usually say no, and I tell them that it just gets annoying. But if there is a legitimate question about what I saw, I have no problems explaining a call to a captain that asks. I also make sure to tell them that there might be a time or two during the game that something might happen that causes me to get blocked from a call, and that it happens sometimes, but that it will not be intentional in any way shape or form.
I find that being honest with the kids makes it a much better game in the long run.