As a principle of law you can stand in the street and take photographs. Obviously its way more complex, and subtle, than just that.
One (of the many) things you have to consider is your right (be that legal or moral) to photograph other people. In fact documentarians and street photographers may go so far as to consider it their duty to photograph people.
But what do the photographed have to say about this irrespective of your rights or duties?
In the UK we're pretty lucky as most people are generally fine with being photographed; but really I suspect that's because they never expect to see the results. If they were to believe they may wake up one morning plastered across billboards, they may not be so accommodating.
In a public space photograph who or what you like (current terrorism legislature allowing). But if you feature people as obvious subjects in a way whereby they could be recognised you need to be careful with how, and where, and if you publish the image.
And these days, you should know to speak to the parents before shooting kids.
I don't feel anyone in this shot would have thought to complain if they found it here one day, so I have published without any permissions, in this instance.
When it comes to candid shots of people, I think it's a case of 'use not abuse'.