Firstly I would get a net nanny pronto and keep an eye on her using the internet. I would ask her straight up about the site, how she found it and why. Then tell her you don't want her visiting it anymore as this is site for grownups. She may have accidently come across it not been actively seeking so be calm and cool about it.

first of all you should block all sugjestive sites and maybe even take away the internet. My kids are only allowed on when i am right there. dont worry, this isnt cruel because the internet isnt a good place for little kids anyways.