GET RID OF THE HUB!!! Hubs are ancient and will suck the life out of any potential speed too. But if you insist on using a hub then you might want to look into software solutions for each PC. One possible solution might be from Symantec. (But it's still not going to restrict new PC's on your connection.)
You might also/instead want to consider setting each PC with a hard set DNS service like OpenDNS (http://www.opendns.com/). With OpenDNS and possibly others, you can filer content and rely on them not returning valid IP lookups for x-rated web sites, for example. But with recent browser bologna, it's still possible to bypass this if your browser predicts web site addresses or especially if you edit your own hosts file.
Therefore, I suggest getting a "router" and then try it's built in functions. Often filtering connections by MAC or IP address is enough. Even better, get a router that can be reprogrammed with the DD-WRT (http://www.dd-wrt.com/site/index) or Tomato software (http://www.polarcloud.com/tomato) for even more functionality.
But perhaps the best solution with kids is the old fashioned technique - WATCH THEM!!! Nothing is better than keeping a kid safe by keeping them honest. And requiring a kid to do things in family/public environment when using a computer is probably the best way to do that. They're not as likely to encounter an online pervert or something that way too. (I'm sure you've heard the lecture. But if you haven't, just thought I'd say it.)