Taking this point by point...
"DS1 (yr 6) got badly told off for bouncing a ball while he was walking to the assembly line and the ball was confiscated for two days."
Possibly not unreasonable to object to this. He should have been told to put the ball away first, and then only upon failure to do this should the ball have been confiscated and the reprimand given. There is no point playing hell with kids over rule breaches if they aren't aware that they are breaching any rules.
"DS2 (yr 5) got into serious trouble for flicking a pencil during a lesson."
This is patently silly behaviour in a classroom and you can expect any kid to get into trouble for this.
"DS1 got shouted at for talking during a lesson and for daring to say that other people were talking too."
This is a situation in which the teacher has lost control of the classroom and has resorted to picking on random talkers in a doomed-to-fail attempt to re-establish order. Having said that, "daring to say that other people were talking too" counts as backchat and will be punished by any teacher anywhere, regardless of whether or not the comment is true.
"Also they tell me that one of these two teachers refers to some other children as "idiots" and the other one is shouting half of the time and almost always at boys only."
Referring to any pupils as idiots is unacceptable, yes. And raising one's voice on a regular basis is pretty much the opposite of good teaching practice.