I'd like to think I'd stand up and do what I could to protect the child. I'm pretty sure I would, however I'm often on my own with my two little ones, what good could I do then? If I was on my own I would step in, I did when I saw a woman smacking her child in the park. She was going over the top and I acted completely out of instinct, I honestly couldn't hold myself back so I expect in a situation like this instinct would kick in.
I think the best answer would be to call the police, whilst with the child, and get them to deal with it. A true abductor would almost certainly run off whereas a parent would stick around (I expect!) and hopefully understand- and even appreciate- the situation you'd been put in.
If the 'dad' seemed very plausible and the child agreed it was his dad but I was still unsure then, at the very least, I would take note of descriptions and car reg etc.