It's a really good topic to discuss, I'm glad you posted it. Here's my take:
Basically, you aren't going to be able to stop kids from ever seeing naked people. The way society is now, almost every kid has several devices that can connect to the internet. They are going to watch porn at some point, whether they intended to or not.
So you can try to block every device they own from accessing any site that can be deemed bad. You can pray they don't have any friends that would ever show them this content. You can campaign that porn is evil and try to get it removed from the internet. But none of it that is effective. It comes down to this:
Talk to your kids about sex. Talk to them about these images mean. Your little boy sees a vagina on the internet? Don't ignore the subject and try to block his browser from every accessing it. Make sure your child knows what you can and can't do. Don't let the internet tell your kids what sexuality is, man up and talk to them. Just because you didn't get "the talk" until you were 15 doesn't mean you have to wait that long for your kids. Accept the fact that today's generation has MUCH easier access to this type of content, and face it head on.