It's pretty good, but the, the Google SafeSearch isn't bad, so I'm complimenting Google as much as the KidRex people. I was slightly disturbed at finding the Martin Luther King racist website in the listings though, so Google/KidRex might want to take a look at that, though to be fair I did need to do a very specific search (Martin Luther King examination) to get there.
The site also takes the very easy way out of simply blocking words. Woe betide the child that wishes to find information on blue tits as a result since it's just not going to happen with this resource! However, this engine passed all my usual 'naughty' word searches, so it's perfectly fine from that aspect, but in all honesty it wouldn't make my top childrens search engine list.