When I was 12, I spent the night with a girl who was 13. She was a regular leg shaver and talked me into shaving. I got one leg done and felt so guilty because my mother hadn't given me permission. I refused to let her do the other leg and worried my mom would be mad at me to the extent that the girl's mom called my mom with a head's up.
When I got home, my mom wasn't mad. She just said, "Silly girl, get in the bath and shave the other leg".
But, you know what? It may sound gross because I'm 50 now and I don't regularly shave my legs. I'm blonde, my arms are practically hairless, and my legs might have 10 hairs each, which you can't even see unless they are gleaming in the sunlight. I do shave about once in the summertime and usually cut myself which is far less attractive.
I personally think that shaving too much and starting too young makes for spikey hair growth as you get older. I've managed to avoid all of that.
If I was hairier than I am, it might be different, but I have no need to shave above my knees, thank God. I rarely have the need to shave below them either.
Depending on how hairy your daughter is, waxing might be better.
I just think that shaving above the knee might lead to needless chafing and irritating the skin.
Just my opinion.