As both a coach and a parent, I think I can see both sides. There's only been a time or two when parents came to me about their kids playing time. And both times, the parents when ballistic cursing, yelling. I kept my calm, told them the "facts" as I saw it, and said my position didn't change. Didn't hear from them after that, and the kid has been busting his butt ever since (similar issue as cadad). As a coach, the parents know they can call me and we'll talk. Not a problem.
As a dad, I've bitten my lip twice now. In his middle school game, the coach let his #1 pitcher go a complete game throwing about 150 pitches. I cringed. I mentioned something to the parents, asking if they kept pitch counts. They just shrugged, so I let it go.
But if that were my kid on the mound, I would certainly say something to the coach.