A few of my clients asked me of the same thing, my answer is always the same:
While there's commercial software that would do the job, note there's no 100% anything when it's a battle between a can and a can opener, be it security, spam, anti-virus, or access control. Most of these Internet access control software uses filters based on key words, site addresses, etc. that users have to update. Problem is many sites don't contain such keywords, and new ones pop up all the time it's impossible to keep track of all of them.
In the end, even if there was such a software that could successfully do it, your kids can always access whatever they want at a friend's place or find ways to disable such a mechanism, and you can't protect them from it forever. The best thing to do is to educate them and instill values in them, expose such things to them so they won't go out of their ways to get at them just because it's prohibited at home, we all know how fun it is to do things we're not supposed to do.
Kind of like parents trying to protect their children from hearing cuss words, they're going to hear it elsewhere anyways so it's ultimately futile, but if you properly guide them and educate them they'll know the difference.