There is no way to filter the web access via Safari directly on the iPod touch or iPad. However, there is a way to create a safe and secure web browsing environment on iOS devices. You will need to disable Safari (and YouTube and the App Store) using Restrictions - a built-in feature in iOS - and setup an alternate Safe Browser like the one from Mobicip.
Please see the tutorials on the Mobicip helpdesk for step-by-step instructions. In addition to setting up a Safe Browser, you can also restrict at the router-level (using OpenDNS, for instance) as Peligro911 has suggested.
Hope you find this helpful.
Disclosure: This response is posted by a Mobicip representative who may stand to gain indirectly from it.
By using OpenDNS to filter your surfing and locking down changes via the "restrictions" section you can control where and when your kids use the Internet on their devices. Google:" OpenDNS Parental Controls", I'm not allowed to include a link cause this is my first post.
It also includes black lists you can add to and white lists so you can let sites through if you choose to. The paid ersionalso includes reporting so you can also see id your kid attempts to access a restricted site.
My sister is using it for her 13 year old and it seems to work for them.
McGruff SafeGuard (remember: Take A Bite Out of Crime) released a Child Safe Browser app for iPhone/iPad/iPod Touch that is a look-alike for Safari, but provides a parent with full control of the categories of websites that can be visited.
It also provides a summary of activity to the parent via email.
Search for McGruff SafeGuard in the App Store, or
check out McGruff SafeGuard video in YouTube