People just don't want to be wrong and accuse someone- we still live in a society where family privacy is ingrained in our minds. It needs to be clearer to people that it is better to call it in and be wrong that to not call it in and be right.
Or just go over and knock on the door, unless there's reason to believe the place is so dangerous you might really be risking your life to do that. Responsible parents who are just dealing with a hysterical, tantrum-throwing child, or a sick child screaming in pain from an ear infection or colic, have nothing to hide, and are likely to be relieved to have someone know they aren't torturing the child. As for the sort of parents that are actually torturing a child, sometimes waiting for police to get there to check on a report means waiting until it's too late to prevent death or serious permanent injury. With borderline parents, who don't really intend to hurt the child, but have lost control of themselves in anger, being interrupted by a visit from a neighbor will both interrupt what they're doing, and force a realization that they need get themselves under control, before visits from concerned neighbors are replaced by visits from police and child welfare officials.
I was once alarmed by the sound of a little girl screaming in an apartment in the building I was living in, fairly late at night. It sounded *really* bad. The child (about 4-5 years old) and her young mother had only moved in with the grandmother a couple of weeks earlier, and I'd never heard the child crying at all before. So I went and knocked hard on the door, asking "Is everything okay?" The grandmother (who was a bit of lowlife, but not dangerous) wasn't home, and the young mother was clearly in a very aggravated state of mind, but she understood immediately why I was there, and was quick to open the door wide and point to the child, who was clearly not injured or cowering in fear, and sort of shout at me "See, she just wont' quit screaming, I'm not doing anything to her!" Indeed, the little girl was just having a full-blown tantrum, standing on the sofa launching more earsplitting screams as her mother was at the door talking to me. I thanked her for reassuring me, and went on my way. Saw her in front of the building with the little girl the next day, girl was all smiles, and I touched base with the mother in a friendly way, to make sure there were no hard feelings.
If I ever did something like this in neighborhood of freestanding houses or row houses, I'd be mentally prepared to run and call 911 at the first sign of aggression from an adult. And I'd also be packing my trusty KelTec P32, as I always am when I leave my house, both for my own protection, and in case I saw something that simply couldn't wait for police to get there (e.g child getting head slammed against a wall by an adult).
When push comes to shove, I firmly believe that WE have a moral responsibility to intervene directly. There have been way too many cases (apparently including this one) where neighbors/relatives/teachers *have* called police or child protective services, figured that's all they could do, and the child ends up dead because police and CPS drop the ball.