Would it help you to know that many of us who didn't have the same experience, also get a "knot in our stomachs" when we see child abuse? My MD has a poster in the office, of a child sitting in a car without the seatbelt, and it says something about the most common form of child abuse. Neglect... none of it is right or good for a child!
Becoming a bit proactive might help, IDK.
If I'm in a store or in public anywhere, and I hear or see a parent "abusing" a child... yelling, jerking, emotionally putting the child down I get involved. Usually I say something like, Wow you have your hands full today, don't you? Something to take their mind off the child and onto their own behavior (yoo hoo, someone saw what you did.>>) I might say, is she/he not feeling well today? And at times, like when a man (father?) slaps a child, I step right up and say, WHOA... that's not allowed... of course, staying far enough away so he can't strike me. But it's that important, to me.
I have called police... that's what we, the public must do. We don't have to SAY there's abuse... but if we're pretty sure, we need to say they need to check things out and see if that family is doing ok.. ...
Once you are no longer overwhelmingly triggered, you will be a good advocate for another child, somewhere, I'm sure.

Take care of yourself, first, though. I hope that knot lessens for you.