I agree with Bill, I think it's an amazing commitment that all of the actors and actresses made to do these 8 films. I think part of it may be that the adults, at least, recognized the vision in the story and wanted to be part of it. Plus, I'm guessing that guaranteed long term work isn't bad either! Although, I doubt you'd find too many American actors that would do the same thing. Which I think is a testament to the dedication the British actors have.
On the topic of careers for the main three, I've wondered what they'll do after the Potter films are over.
All three of them have definitely gotten better as the films have gone on. I think Radcliffe has the most straight forward of the three roles, even though Harry has the biggest role in the movies. Rupert may actually be the best actor of the three, at least by a little, and due to the way the character has translated from the novels. Emma is right there with him and those two, IMO, probably show the most range of the three.
I think Emma won't have a problem getting roles, she's hot and I'm sure a lot of jobs are going to be thrown at her just because of that. I believe Rupert has already done one or two other things, which I haven't seen, and if he does well there's nothing to stop him from having a good career.
I think it'll take a long career of really good roles for Radcliffe to overcome his always being thought of as Harry. He'd need a Harrison Ford type career with a lot of varied characters in different genres to do it. Otherwise he becomes Mark Hamill, forever known for that one role, for good or for bad.