I'm tempted to say Britney is not a child star. She debuted with ...Baby One More Time and she looked like an adult there. And she was not famous when she did the Mickey Mouse thing.
She was a child star. If this discussion was about the best childhood star, you'd say it was Britney.
I wouldn't consider her a child 'star' either. Alexz is right - she wasn't famous when she did the Mickey Mouse club thing. Miley Cyrus, for example, is a child star because she was famous during Hannah Montana.