so what would you do?

I would approach them and ask if everything was all right. I don't think I am physically brave, but I wouldn't hesitate if a child seemed to be in danger. I think most people would fear embarrassment but that is just the price you have to pay for looking out for children.
I can never forget the woman who saw the two boys abducting Jamie Bulger and accepted their statement that they were taking him to the police station because he was lost. She has to live with her failure to take action.

I would like to think that I would be brave enough to do as your friend did, but it's hard to say. I think a lot would depend on how the man looked, and if there were other people around for backup, if it were needed.

I would approach them as your friend did. Better to check than live with the guilt if .......

Thing is, if you asked him, he would lie anyway. He's not going to admit it, is he?
So what do you do then?

Small, screaming children often don't tell you what's wrong, and you're just another stranger.

I would do the same as your friend I think.

A friend of mine was reported to the police when he was in the park with his grandson who was about two years old. The child didn't want to get off the swing and there was a bit of an altercation with much shouting and screaming. When the police arrived the small child, who hadn't been talking for very long, refused to have anything to do with them and my friend couldn't prove that he was the child's grandfather. Consequently, both were taken to the police station. It took ages to get hold of his wife to come and sort out the muddle. Fortunately, he's quite a philosophical man and reckoned that it was "better to be safe than sorry".

I would have intervened. I know the odds are it was harmless, as indeed it turned out to be, but when a child is involved you have to be careful.

My youngest daughter used to have the most terrible silent tantrums where she would just lay prostrate no matter where we happened to be. It was impossible to move her...how do toddlers suddenly manage to increase their body weight tenfold? So I would find a wall or something nearby to sit on and wait. What surprised me was how many people walked passed this apparently unconcious child? I rather liked it when people did stop and ask because at least it showed that they cared and also gave me someone to chat to which helped to pass the ten minutes or so that she would lay there until she became bored at the lack of attention and would get up as if nothing had happened lol.