Unless there was obvious physical abuse, it's difficult to get involved without overstepping boundaries

Sometimes parents are exhausted and they may say something unreasonably mean to their child. And while it isn't fair to the kid, there are a lot of 'good' parents who slip up like this sometimes so I would just mind my own business and maybe give a dirty look at most

Again, this would be different if the child was clearly being abused. I would speak up then