NTA , there are so many temporary dyes and hair sprays that children can experiment with. It’s fun that OP’s daughter wants to express herself and be unique, but a 9 year old in elementary school does not fully know proper hair care techniques for regularly bleached hair. Hell, I’m still figuring out hair and skin care that works best for me and I’m mid 20s now. Damage is not always reversible and OP isn’t an asshole for wanting to be safe rather than sorry. Temporary dyes, stick on nails, and clip earrings were all safe and fun things that I played with growing up, and I understood no problem that the real versions of those things were mainly intended for adults.