Not a parent, not planning on being one, but this is how it would likely play out.

Since your situation isn't specific, I'm going to assume I'm there with my daughter and she's clothed, as we're walking down the street or something. I'm also goign to assume that you think people doing this aren't normally being discreet.

I'd probably respond with a "What the hell are you doing?" type thing. He likely wouldn't answer(or not give a good reason), so there would probably end up being some sort of public spectacle of me trying to take the camera.

If you choose not to do anything about it, when you're aware of it happening, you're just stupid.

Now, in a more realistic event:
You find out someone has posted pictures of her online, and she was alone at the time they were photographed. The parent(s) would likely attempt to take legal action in such a case. Reason being that such an action seems like that of a pedophile. Her pictures are already forever a part of the mess that is the internet but taking such action helps to show you'r e acaring parent and also helps lessen the blow.