When I lived in Sacramento in the 1980s, the man across the street beat his kids. Not brutally, but he'd lose his temper all the time; and he'd hit them a lot, yelling terrible things at them. Sometimes, I did quiet things to intervene, but I never reported him to the police. I figured it was his wife's responsibility to do that. Everybody in the neighborhood knew he was beating his kids, and I was the only one who even made a feeble attempt to intervene - and believe me, my attempt was feeble.
You watch something like that, and you don't know what to do. You're half afraid that calling the police will just make him madder and more likely to harm a child. And he and his wife lost a child to drowning early in their marriage - was that the cause of his anger, or was the death of his child really his fault somehow.
All I can say is that I saw it, and I didn't know what to do. Eventually, the wife divorced him and he moved out of the house, and the neighborhood quieted down. But his angry outbursts were a disconcertment in the neighborhood for a long, long time; and nobody did anything about it.

Physical abuse of children is a very obvious thing. Sexual abuse isn't. It often doesn't get reported for years; and people really aren't likely to believe a report of a five-year-old incident if it wasn't reported right after it happened. It's human nature to deny things like this. If you don't see it, the tendency is not to face it. Even in very obvious cases like the child abuse I witnessed, it was hard to know what to do.

-Joe-