I wore those at age 8/9, but only because I truly needed them. I hated them because I was the only one in my class wearing a bra..
Spoilerby age 10 I was wearing a 30B, by age 12 a 32C and now a 32DD. Funny how my band size hasn't changed in 11 years lol
There is a certain degree of peer pressure when it comes to young girls and bras. The only reason I hated them was because noone else was wearing them, not because they were uncomfortable or anything because they weren't. In your daughter's case, the only reason she wants to wear them is because her friends DO wear them (regardless of whether they need them or not). The good thing here is that when she starts to develop she probably won't have a hard time adjusting to having to wear a bra for real.
I personally think it's perfectly fine, but then again it's not my kid. As long as it's cute and girly and not sexy, then why not?