Sorry, the article I read described it as a pornographic exhibition. As for your second point, it doesn't make it okay because her mother authorised it, she is now a grown woman trying to supress the image. (posted to Tate Modern removes naked Brooke Shields picture)
On moral grounds I agree that Shields has the right to try to back out of the contract her mother signed, or to buy the rights to her own image. This is the problem with parents who use their children (posted to Tate Modern removes naked Brooke Shields picture)
The problem here isn't that the image was to be displayed in an erotic art exhibit but that it was taken at all. No amount of fame or fortune should be enough to make a parent allow her child to (posted to Tate Modern removes naked Brooke Shields picture)