For restrictions on the iPhone you can go into the settings and do restrictions on a lot of things with making passcode. You can restrict content safari downloading apps etc. I don't think I answered about what you requested but I wanted to be helpfull. It's under settings general restrictions.
Actually, yes, you can with an app in Cydia called GuizmoDNS. I've done it an now use Google DNS service or you can use OpenDNS's.