@littlek,
I think I told this story on the thread that littlek is referring too -- but when my kid was in preschool, we were hanging out with other kids on the playground after school when one proceeded to have a tantrum. The mom dragged her over to the car and put her in there, windows up and doors locked. (The kid didn't know how to get out or there was some sort of child-proof element, I don't know.)
It was a pretty hot day. The kid was hysterical. I kept an eye on her and I didn't like the look of things. The mom was pointedly avoiding looking at her daughter, instead paying attention to her other kid who was still playing. Time passed. I did NOT like it and started giving the mom looks, like "OK, that's enough now don't you think?" She eventually noticed but just glared at me.
This continued until I couldn't take it any more and confronted her. I forget how it went exactly but it was something like "I totally get that you felt the need to discipline your daughter and I don't wish to interfere in that, but don't you think it's too hot of a day to keep her in a closed-up car for this long?" At the end of it, she huffed and got the kid out. The kid was flushed and did NOT look good but recovered.
The kid now goes to my daughter's school and whenever I see the mom she STILL glares at me. I second-guess in both directions. (Should I have done something sooner? Should I have let it go longer before interfering?)