In general, I think that there are more courteous parents than parents who are not courteous to other travellers. I've had some pretty negative experiences with kids on planes, but at the same time there are many flights that I am on that I don't even realize there were young children on the plane until I'm getting off.

However, I do think there is a subset of parents who really do not care how their or their child's behavior affects other passengers. They aren't even thinking about the other passengers on the plane. Which why we have kids playing in aisles, kids running around terminals, diapers getting changed in seats, etc. It stinks that a small group of parents who act as if everyone else around them should cater to them, when I think the majority are generally pretty courteous, or at least courteous enough that I don't even know that their children are on the plane.