I think the issue boils down to whether kids will accidentally come across porn - the author presents a very misleading example of a search result for 'male human'. When I search for that I get a high-quality porn-free page: http://en.wikipedia.org/wiki/Male_human.

If kids are actively looking for porn, they are going to find it. Now, it would be nice for public schools with strict white list based domain filters to be able to include Wikipedia as a safe site.