I suppose this is one of those topics where everyone must follow their own feelings. I personally do not think it is right to use long telephoto lenses to take candids of people in public places without the persons knowledge. If it is done then the photographer should go to the person, explain, show them the photograph and ask if they want it deleted. To take under-cover shots can only increase peoples paranoia of the camera and who can blame them. There is a long tradition of street photography with Cartier-Bresson leading the way with his shots taken by shop windows etc. but times are different now, though he used a 50mm lens he did conceal the camera at times. To use a long lens would make me feel like I was sneaking about doing something I shouldn't. My opinion is ask first, preferably, if not before then definitely after the shot. With children I would say a definite must ask the parent before. There are plenty of subjects who will be only to willing to have their photographs taken in the street/public places. Usually people who are doing something eg skateboarding, kiting etc. will be only to happy to have their picture taken especially if you offer to send them a copy. Failing that hire a model send him/her out shopping, in the park, etc. and you can take all the candids you want. Only my personal opinion, not what the law says, or what anyone else has to think.

And every one said, 'If we only live,
We too will go to sea in a Sieve,---
To the hills of the Chankly Bore!'
Far and few, far and few, Are the lands where the Jumblies live;
Their heads are green, and their hands are blue, And they went to sea in a Sieve.

Edward Lear
http://www.nonsenselit.org/Lear/ns/jumblies.html