When Do You Say Something?

I am VERY much a guy that believes in minding my own business. But, a couple of incidents have me wondering when it's OK to say something to the parents of a child that isn't yours.

First was the video of the buffalo tossing that 9 year old girl in the air in Yellowstone Park this week. In the video, you see two adults running away first, not trying to help that poor kid. REALLY?

Then, in another incident at a shopping mall I witnessed a dad telling the crying child in his arms to "shut the expletive up". It really made my blood boil, but it wasn't my child.

So, when is it OK to say something to these kinds of people?