In the last area we lived, I was personally involved with a few kids who were abused to varying degrees. (Different families, different types of abuse, but all messed up situations.)
How I handled it was on a case by case basis. One girl just wanted an adult woman to be her friend. Her mom walked out on her family a long time before and she had never had a stable woman in her life. Additionally, there was abuse happening, I'm certain. She never came right and accused her abuser to me and I never really had any concrete evidence that she was being abused so I didn't feel like I was in a position to do anything about it legally. But I did have strong suspicion of what was going on and that it was actually happening to her on a fairly regular basis. Another girl actually was walking past my house barefooted and I brought her inside and called someone who could help straighten out the situation. She chose to stay at my house for a few hours that night, I fed her dinner and took care of her until the situation was diffused enough that she felt comfortable going home. I was friends with her mom and did what I could thru that friendship, but I always felt like there was only so much I could do. I mean, I'm not a social worker, I'm not a cop... All I can do is be supportive and try to keep a positive friendship going on so that they don't slam the door in my face too. I felt a bit of responsibility where I had the information that I did to kind of keep tabs (particularly where these girls were seeking me out) in case something really bad started to happen. I wanted them to feel safe at my house if they needed someplace to go in a hurry.
There were also a few other families I knew of where neglect was going on and in those cases, I tried to befriend the kids, but more than that, I tried to befriend the moms. (In the neglect cases, it was pretty much 100% single-family households where mom was busting her butt to make a living and other responsibilities were getting lost in the shuffle.) I tried to support them emotionally, but again, there's only so much I can do as a friend. Sometimes I'd have the kids hang out with me for a while, but I couldn't really be a doormat either. I'm not a free babysitter.
Knowing about abuse is a difficult place to be. But I never felt like I could just turn my back and pretend it wasn't happening. I have kids, and it ripped my heart out to think of what if's... What if my kids had ended up in a different household with different parents... without feeling loved....? I just wanted them to experience a little love, and a little bit of "normal" (well, if you can call me normal, ROFL!!! :bouncy: )