@Linkat,
Hmmm...I have certainly intervened in a lost child situation...something still a bit easier for women than men, I think.
If I saw a toddler darting away in a situation which placed them in danger, and a parent impeded and unable to do anything, I would intervene too.
It's a hell of a judgment call, though.
It's awful when you see an interaction which clearly sees a parent out of control, that stops short of abuse so clear that I would call the cops.
When I see a parent trying to manage a tantrum in a very public place I always, if I easily can, catch their eye and give them an empathic glance....young parents, especially, here are very subject to what I call the Supermarket Mafia...where older people are very open in their disapproval if a little person is giving trouble.
One of our staff at work grabbed a toddler who had made it away from their parents, down a long corridor, and out automatic doors, onto a major road with heavy traffic.
The parents were resentful and rude (probably because they felt bad) when the little fella was returned. Poor kid.