Legality and decency are, often, too far removed from each other.
If someone were taking photos of my child, I would ask them not to. If that person persisted, I would do what was necessary to ensure that the photographer was unable to continue, and which would likely make him wish he'd simply agreed to stop shooting in the first place.
If that resulted in me having to deal with the authorities and legal ramifications related to my actions, so be it; wouldn't be the first time.
That photographer would, however, most definitely stop shooting.
Can't say I'd disagree with that line of thinking, but that's quite the leap to go straight from "asking nicely" to using violence. Obviously you can do other things first, like making the case that you'll get the cops involved, or obstruct the photographers line of view. If they're actually doing anything questionable, they're not going to be interested in starting a confrontation. Violence really should be the last resort--even moreso when you consider that many photographers are carrying firearms, knives, etc. for protection.
I've had people get belligerent with me just because I took photos of a crowd at a large event, and they happened to walk into the frame. If people start getting physical with me--whether they think they're in the right or not--I'm going to defend myself to the best of my ability, and that can include the use of legal weapons. I really cannot imagine ANY situation where it should be worth having someone end up in the hospital or jail over a picture.