It's bollix of situation really, I was at game a few weeks ago and there was a minor incident in the game with a player being a bit late with a tackle (U12) but one of the supporters kept shouting at the ref (fairly abusive and foul mouthed at times) for the next 15 minutes over it and every free from there.
A m8 of mine walked over to the side where he was standing and quietly and discreetly had a word with him, not a big scene or anything. It worked and it stopped, afterwards the opposing mentor thanked my m8 and said it was an ongoing issue with the same parent.
If a club can't control supporters at kids games then they should be made wait outside the ground. It just spoils the enjoyment for everyone else.
There is no way I would tolerate it from parents at our game, they have lost the plot if they think it is acceptable. In the end kids on the pitch do silly things at times and it gets dealt with by the ref or the coaches. Roaring from the sideline about it doesn't help anyone, least of all the kids who are the subject of the angst!
My own young fella got absolutely buried yesterday morning with a slide tackle straight into his standing ankle, 100% intentional and left him in tears hurt. The tackle was from a young fella completely frustrated because they were being well beaten, no point roaring and screaming at anyone as the young fella just lost the head, its not his managers fault and he will learn himself hopefully.