Parental control or outbound firewall?

I would highly recommend using OpenDNS as a first point of call. You may find that it blocks out a great deal of the crap that you don't want your kids to see. It's quite feature rich, but applies the rules to anyone who uses your network/router. I wrote about it a little while ago. If you're interested, check out the post here.

If you want a per-machine solution, perhaps youc an look at something like NetNanny?

Good luck!

OpenDNS is certainly one option and probably the best option if you don't want the hassle of installing and configuring some software.

If you are willing to do some installation and configuration you should take a look at DansGuardian which will allow you to do the same kind of content-filtering as OpenDNS with a little more granularity (you can, for instance, dictate which users or IPs should be content-filtered and how .. and even whitelist specific URLs).

The normal mode of operation of DansGuardian is that it works in conjunction with a proxy. If you have multiple machines on a home network, you can install a proxy/firewall to be used by your entire network. If you have only a single Linux system on which you want to do this, you can install a small local proxy in conjunction with DansGuardian (there are a number of tutorials out there with instructions on how to do that).

If you are building a DansGuardian proxy/firewall - you can either roll your own or take a look at using one of the firewall in a box type Linux distributions. Two well known and widely used distributions are IPCOP (with the Advanced Proxy and URL filter add-ons) or Endian community edition - both of which provide nice web interfaces for the configuration and management of the firewall, including the content filtering. They can both be configured to download updated lists on a regular basis from various sources.