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Do you know which child in particular is looking at the nude pictures, or do you suspect it's both?

I have no suggestions on how to block sites, because honestly, that stuff is hidden under the guise of anything. There may very well be programs out there to do it, but I don't know about them. (My son isn't in an age category where I have to worry about this yet.)

If it were my house, I would have a talk with the children about why this bothers you and why you don't want them looking up that sort of material. (Though honestly, they're at an age where they're curious and I would argue this is healthy behavior for the age. Like the stereotypical Playboy under the bed scenario.) Also, there would be no computers in secluded areas: all out in the open where they're easy to moniter. Your children are less likely to visit sites you don't approve of if they know you can see what they're doing.

I agree that this is something you definitely need to talk to your kids about.

As far as blocking programs, you can run a search in Google, such as "blocking graphic content". The paid ads at the top should yield programs that will help. Also, Download.com has some programs with high user rating for monitoring your children's internet usage and blocking inappropriate content.

I hope that helps.

At that age, kids are curious about that sort of thing. But, you should definitely talk to them about it.