girls should not dye their hair until they hit puberty. your hair gets damaged enough over the years and she should wait. she will thank you in the future. what if she doesn't like it she will be mortified.
Depends on the color she want's. Nothing relating to the colors of the rainbow. I think maybe blonde highlights would be nice if she wants something different with her hair. But nothing like blue or green or pink or anything like that.
The integrity of the hair can be permanently weakened if you color or perm it before reaching puberty. If that is not an issue then my advice would be to take her to a professional and don't try an all over color change at home.