I will let others give you their own methods and ideas, but I just wanted you to be aware, if you are not already, about search engines.
For example Google allows searches in various "Safe" modes.
Off - No filters and 'anything' works. For example, searching for 'explicit' web sites will not be filtered out, and even searching for innocent search terms may turn up explicit web sites and other results.
Moderate (Recommended) - excludes sexually explicit video and images but innocent search terms may still result in unwanted listings.
Strict - filters sexually explicit video and images from Google Search result pages, as well as results that might link to explicit content.
I got those details from http://support.google.com/websearch/bin/answer.py?hl=en&answer=2519950&rd=1 and that might help you more.
I am unsure how you can prevent users from resetting or changing these filters.