My daughter is 8. She's fine to wear short-shorts if she's doing it because they're comfy and it's hot.
She's not fine to wear them if she thinks it's important to make sure that the general public catch a glimpse of her inner thighs on an outing to the shops.
Same shorts, different reasons for wearing them.
Of course, I'd not ever let her stroll around town in a pair of shorts that revealed her buttcheeks regardless of intention. Some things just aren't appropriate to wear in public, especially when you're still a child.