If it should be fine for a grown woman to go topless, then shouldn't it be fine for girls of all ages to go topless, if they want to?
And wouldn't that be the best way to normalize women going topless, by letting them start as kids? Most kids aren't as insecure as adults are, and I knew several girls when I was younger who thought it was unfair even before we were 10 years old.
I'm not saying we should do this, but if women wanna have breasts be less sexualized and going topless more normalized, I think "grandfathering" a generation of girls into it would make the most sense.