The Internet is not a safe place for kids
We all know of content we don't want our kids to be exposed to, be it either accidentally or on purpose. We therefore have to do something about it. There are different approaches to gain some security but all fail when it comes to the details. Let me explain why:
Allow list / Whitelists
Whitelists securely block unwanted content and can be generated by a variety of browser plugins or parental control software but they will not last for long. We want our kids to discover the world, to learn how to operate the Internet, to learn how to find information, and to learn how to play games that are safe for them. They will not learn how to do this if they only have access to a small list of sites granted by Daddy where the next click on a button leads to the "BANNED" page. Only very young kids may be happy for some months with a whitelist.
Deny list / Blacklists
Blacklists such as offered e.g. by DNS services are meant to contain all known bad sites and block them. This task is ridiculous. We can not possibly know of all bad sites. They pop up everyday in thousands. The makers of Dansguardian filters put it like that:
The web is a fast changing place and even large web search engines such as Google or Altavista or Yahoo don't even know of half of it. This makes filtering by web address (URL) difficult as sites change and new ones come up all the time.
To overcome limitations of a blacklist we additionally need some sort of content filtering but it is tedious to maintain the filters. This may be good for schools when a full time employee does all the work but it is unlikely we find the time at home to do so. Using pre-made filters is likely a bad idea as the bad sites know about them and avoid those suspicious phrases to appear on their pages (look at the spam mail you get and you have an idea on how they do it).
As already mentioned kids become smart, and we desperately want them to become so. Unortunately then they will also learn how to overcome most of our filters. They will (yes, they will, no matter what we do) gain access to anything they want by the time they are smart. All our blocking efforts will fail by then. Before that however they may not be interested in all that adult stuff at all. So there would in reality not be a need to block anything. By the time they are smart they need to be smart enough to know what is a bad site, and they should be smart enough to be interested in other things rather than visits on adult pages.
We all are or will be smart parents. We want to teach our kids how to operate Ubuntu and how to discover the Internet. Smart parents watch what their kids do and they talk to them about what they do. We should provide them with the knowledge on what to do when they come to sites that are different or may be harmful. We want our kids to tell us what they do. Smart parents spent the time they would need to set up and maintain a parental control rather with their kids. We also should not leave the kids alone. Put the computer to a place where you can pass by and have a short look what is displayed on the screen.
In summary the "best" way to refrain our kids from visiting unwanted sites is to become a smart parent educating their kids to become smart. But I know from my own experience that this also may fail, has inconsistencies, and may likely not achieve 100% safety. It still is the best.
Note after seven years: We never had any content filters in all those years. I can see now how important all I said above was to our kids.