What would you have done?

I'd have taken the car reg number and made sure the man saw me watching.

I once stopped a man who was dragging a small child through a shopping centre, the child was screaming he didn't want to go and wanted his mother. Everything was fine but I was shaking and very nervous but was still very glad I did.

Being quick enough to find your phone and take a picture is unlikely and could get you in trouble if it is innocent.
I only once challenged a parent about their child. A boy was deliberately trying to break down a tree that had been planted a few weeks before. I very politely asked his mother to ask him to stop. I got a string of FFFFFFFF off etc.

Photographs of man, child and car then phone police. Better safe than sorry.
Hopefully it was merely a tantrum at leaving the playground!

I think I might have risked a wimpish"Excuse me is there a problem? Kids, eh. Is this your daughter/daddy ?" (Depending who I was addressing it to)
I hope so.

Very difficult, given the power of hindsight, I think I would take his car reg number and phone the police immediately. 99.99% it was a harassed dad. But, God, how would you feel if it was an abduction and you did nothing.