I wouldn't start a kid with Nair or any of those creams, just because of the possibilities of irritation. Start her when you (and she) think it's a good time, but really unless you have a really hairy kid who finds that to be problematic for self-esteem there's probably no reason to encourage shaving any earlier than its going to happen through social pressure, etc.
(And I know plenty of women, including my partner, who rarely if ever shave).