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.