You are doing the right thing by agreeing to teach your daughter to shave. I had very dark, thick hair on my legs and arms from a young age, and by the time I was in 6th/7th grade, most of my girlfriends were shaving. I was made fun of (mostly goodnaturedly, but it still made me feel insecure and 'uncool') numerous times for how hairy my legs were. If I had waited until after my "first menses", as some pediatricians suggest, that would have kept me not shaving until I was 15 years old. For a 9th/10th grader to not be allowed to shave yet is just setting them up for major humiliation, to say the least!
I also have thick, dark hair on my arms and have had little kids comment on it innocently. Weirdly, my arm hair has never bothered me. Probably the "societal norms" thing as another commenter remarked. I shaved my arms once and it just looked weird to me, so I never did it again.
In Japan - where I lived for 5 years - women shave their arms, legs, underarms, faces, (seriously), eyebrows, upper lips... (Though funnily enough with all that attention to avid hair removal, they tend to largely ignore the pubic region...!) Though many high school girls will not begin shaving until later in their high school lives... Kids stay "younger" there for longer than they do here in the US. Anyway, just another example that societal norms, it seems, play a larger role in the age and method of hair removal than any sort of biological timeframe.
Good luck with the shaving lessons!