I did once take a photo of my dog with two cute little twin 8 year old girls who live in my neighborhood. I told them to tell their parents, and that I wasn't going to put that one up online, and I didn't. Later, I met their father when my dog and I were out on a walk, after I introduced myself, I made sure to tell him explicitly that I had taken the photo. I just don't think it's fair to put kids' photos online like that.
On he other hand, people who are in a hurry or feel vulnerable mostly say no just by default. Does it reflect their true feelings about the matter if they had time to reflect? Hard to know.
In the US the situation should be simple. The first amendment must trump any right to any expectation of privacy in a public space.