I have to say I wouldn't like random people taking pictures of my child, is it that hard to ask for consent, explain what you are doing before hand? I think if they were in a performance or a display or dressed up for something then I would be more lenient however if I am out and about with my child and we are relaxing and doing normal stuff and someone started taking pics I'd be quite angry about it. In fact they catch me on the wrong day of the month they'd be liable to have a camera rammed somewhere unpleasant
How do we as parents know which photographers are the good ones and which are perverts? The fact is we dont know and no pervert is going to confess they are so we have to assume that, unless you ask us and explain why you want a few pics, you could be using our childrens pictures in anyway imaginable and it doesn't necessarily have to be in a perverted way, you could do anything at all with the pictures on your cameras and we will never know what has been done.
My child is the most precious thing in the world to me and I will protect him in any way imaginable and if that makes me a b*tch or an idiot or whatever else you have in mind then so be it.