Google's parental controls are pretty feeble, though there's a roundabout way to lock filtering to the most severe setting.
What about Microsoft's Bing? Bing starts out with one significant advantage -- the connections are not https encrypted so OS X domain filtering actually works.
Alas Bing's controls seem even weaker (emphases mine) ...
... Ensure that SafeSearch is always on when your kids search on Bing, choose what they see online, set time limits and game restrictions, and more. Windows users can install the free download, Windows Live Family Safety....
Ensure SafeSearch is on, I presume, by standing over your favorite teen! [3]
Interestingly in 2009 Microsoft had a better approach to parental controls ...
Bing Modified To Enable Porn Filtering - CBS News
... , "explicit images and video content will now be coming from a separate single domain, explicit.bing.net...
... Almost all third-party filtering tools can be configured to block specific domains or sites, as can the parental controls in Microsoft Vista and Mac OS X...
After this 2009 press release however, Microsoft removed all references to explicit.bing.net [1]. I wasn't able to find any explanation of what happened to it. [2]
My own tests suggest Microsoft really did abandon this scheme. The domain 'explicit.bing.net' brings up the usual bing interface, so the domain still exists, but there's no longer any redirecting to this domain. For example, a search on "Hot Babes" with OS X Parental Controls blocking explicit.bing.net brought up an impressive array of high resolution images.
Without the ability to lock Bing's parental control settings, and with the inexplicable demise of explicit.bing.net, Bing manages to come in 2nd behind Google's parental controls. The one advantage of Bing is that the lack of https encryption makes it easier to track pages visited.
[1] Either that or their feeble Blog search doesn't work with Safari. Microsoft is falling apart almost as fast as it grew to power.
[2] I wonder if corporate lawyers advise against attempting to do parental controls. Microsoft probably has less liability if they do nothing than if they provide an imperfect solution.
[3] Though it doesn't help OS X users like me, Windows Live Family Safety sounds relatively useful.