One thing I would add to what I had said before: Don't give her too much free time. My first time of being free in the parks was special and I wanted more...so much more that the next few trips I barely spent any time with my family. The results are this:
1. My family vacations turned into solo adventures. The concept of family is so lost these days that it is a good idea to make sure she spends plenty of actual together time with you guys, too!
2. There are virtually no pictures of me at WDW once I was a teen...an occasional one here and there when we happened to be in the same park and ran into each other, but that's it.
3. I was so eager to ride my favorite rides over and over that I missed out on some other rides/attractions that I would/should have experienced. Some I was able to do later as an adult, some were gone by then.
4. My best memories of childhood always seem to revolve around WDW. Most of those good ones are the ones where I was with my family. There aren't a lot from my teen years.
This is the reason why i wouldn't even make the suggestion to my daughter to go round the parks on her own (she's 13). Yes i know she would most likely be safe, but i don't feel the need to push her to be independent on a family holiday at such a young age and she doesn't feel the need to do her own thing on holiday. Tbh i don't think she would find it much fun anyway my children (17 and 13) want us all to ride the rides together.