I'd find it hard to believe that this would even be an issue. Are you saying that she might try to navigate to it on purpose or are you worried that pop-ups etc. might bring the unwanted content to her?
A couple things I can recommend...
Make sure the computer is clean of spyware and adware and it has adaquate protection. Microsoft AntiSpyware is fairly decent, it's what I've been using with my clients lately, because it has realtime protection and some other features. Spybot and Ad-aware do the job nicely too. A lot of times it's the spyware and adware that's bringing the unwanted content in, and without realtime protection, your daughter might click away and bring the stuff in. Believe me, it happens even with the elderly people I train who get their first computers. If someone doesn't know what they're clicking, you'll get into a mess quickly.
Second, there are some proxies out there that are meant to only allow decent sites to pass through. They just block out all the sites that are registered in their list and allow the good sites to pass through. You actually configure your browser for a proxy and all the pages have to filter through that first. That could be an even better, and free option compared to net filtering software.
I haven't really used net filtering at all (I'm 16 myself and I've been setting up my family's computers for as long as I can remember) but those are a couple suggestions I can give you.
Good luck!