A week or two ago I was at a sprawling adventure playground in a large city park.

My children were running around and I was strolling around keeping half an eye on them when I came upon a boy, who looked about 10 years old, and a man I presume to be his father.

The pair faced one another; the man had his back to me and beyond him the child was facing towards me. The man held a big stick, almost a metre long, and deliberately whacked the child over the shoulder with it, hard. The boy cried out in pain.

I wasn’t expecting it so I didn’t know in advance what I would do or should do. What I did do was blurt out in shock, “You can’t do that! It’s illegal!”

The man looked at me confused, like he wanted to explain but didn’t have the words. He left with the child, giving him another big whack across his back - with his hand this time - as they walked.

Later I saw the boy standing outside the toilet block, smiling at me.

I approached cautiously, not wanting to get too close in case the man suddenly came out of the toilet. Mostly I didn’t want the boy to get in trouble for talking to me. I asked if he was all right, and he said yes.

I turned away, with the half-formed thought that I could look up the number for the Kids Helpline and slip it to him while his dad was in the bathroom, but I ran out of time to put this plan into effect.

There was a limit to what else I could do, given that I didn’t know anything about them. My friend suggested the park rangers might have procedures, but by the time I found their number the man and boy had left.

I don’t know whether this man is unpredictably violent or if he is an otherwise loving father with misguided ideas about acceptable punishment.

I appreciate the difference but neither scenario is OK.

While there has been a push in recent years to outlaw smacking children, the law currently says parents can use “reasonable force”. I checked with Professor Sharon Dawe, a specialist in child maltreatment at Griffith University, who said that while it was judgment call, using a big stick to hit a child is not considered reasonable.

This wasn’t a harried parent losing their cool, administering a gentle tap on the bum or restraining a child. It was horrifying and it made me wonder if he behaved like this in public what might happen behind closed doors.

What is my responsibility as a bystander to try to protect a child from harm, even from their own parents?

A generation ago, we might have said it was a private matter, but times have changed. Now the rhetoric is that we all have responsibility to look out for children.

“We do have a responsibility as a humane society to step in and mitigate harm,” Dawe says. “Your initial response of trying to intervene and make it stop needs to happen quite often.”

But we are all prone to what psychologists call the “bystander effect”, where most people are unlikely to intervene if other people are present because of social awkwardness.

Dr Philip Zimbardo, the psychologist famous for the 1971 Stanford Prison Experiment that simulated the abuse of prisoners by guards, has more recently been working on the Heroic Imagination Project – asking not how good people become evil but rather how ordinary people can act heroically. The idea is that we can’t all be superheroes but we can all be everyday heroes - not that I'm claiming this title - with the courage to act when the situation calls for it.

But how?

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If the child is known to you then you can report the situation to child protection authorities who can investigate and consider any case history.

In my case, it was a true bystander situation. I had no idea where the family lived or where the child went to school and it wasn’t practical for me to tail them to the carpark and take the number plate like some sort of overgrown Nancy Drew when I needed to look after my own children.

Afterwards I was wondering whether my intervention helped or might have made things worse for the child.

Dawe says you need to “judge each situation in its context”. You shouldn’t stand by and watch a child be beaten but if it appears the situation is over then intervention might sometimes escalate it.

She says if you get the opportunity to talk to the child in private, as I did briefly, it can be worth telling them that they can ring Kids Helpline if they ever need to talk to someone outside the family – it's simplest just to do this verbally and children over the age of seven are capable of retaining that information and following up. For younger children, it's worth planting the idea that it's OK to speak to someone outside the family about it.

A friend who was abused as a child told me that the few times in her childhood when random adults tried to help her were memorable. The fact another person noticed and cared, made her realise her experience was not normal and helped develop her sense of right and wrong.

Even if my response was imperfect, the fact the boy smiled at me tells me I did the right thing.

Kids Helpline: kidshelpline.com.au; 1800 55 1800

Caitlin Fitzsimmons writes columns about work, life and money. Find her on Facebook and Twitter.