None of this is legitimate here without specifying what country it's for, as it varies from country/state to country/state...
This is true, although it holds constant enough throughout the United States. As an somewhat of an amateur street photographer, I have to ask myself this question all the time, or, more often, "is this worth the argument with the individual, the individual's family/friends and/or the police that it may entail?" Often, the answer is yes. While one can shoot anything he or she wants in a public place (including anyone, even emergency first responders), this is not really all that well known. A good parallel is laws related to cycling: Even though legally I must ride my bike in traffic and in the lane of travel (not the shoulder), I still get buzzed, yelled at, and occasionally hit with things for doing so, sometimes even by cops. If you take a picture of a local police officer (especially doing something embarassing) and he or she sees you, you may be in for an interesting ride. The same goes for a child while a parent is watching.
Ultimately, I don't think it's worth letting this stop you from shooting what you want to. I've never gotten more than a dirty look (except once from a girl who was legitimately wasted), and more often that not, people are so wrapped up in whatever mundane task (or three) they're performing that they won't even notice. However, just because something is completely legal doesn't mean it will never get you into some sort of conflict.
Children are, for some reason involving paranoia and the general apprehension of the evil, evil Internet, the most obsessed-over group is small children. Somehow, as a photographer, there is some sort of evil act I can commit with my black and white negatives that will end up in the worst imaginable violations of the child being spread to every single person on Earth through this unimaginably vile transmission medium. For this reason, I ask when I can with children, mostly toddlers. It's not like they're going to pose for more than a second anyway. Sometimes, however, you just have to pull the trigger:
viridari said:
I don't like to ask permission to take photographs. Then people pose. I take the photograph, then approach them and give them a card with my email address and Flickr URI on it and tell them I will give them a copy of their photo for free.
This is a great idea. I am seriously considering doing this myself, although I would lose my Phantom-esque mystery.
If you are not in the United States, this could all be invalid. Laws vary from country to country.