I loved this when I was about 5. I now use normal internet browsers, but this is still very good for kids. I recommend the paid subscription, it's pretty cheap (about $40 per year?) and you can get some pretty cool things. I loved the option to make my own character. And if you watch, say, a video, you can "rate" it by using one of the icons (I remember them being things like "boring" or "cool". I'm sure KidZui has changed since I last used it, hopefully it's still good!
I started using it when I was in Kindergarten, It's great! My sister uses it but now we're used to the plain old internet because there's things WE are allowed to see, which our parents allow us to see, but it's not our parents so it just BLOCKS it. It's not fair, and we're trained not to go on mature sites, so its more like a yeah, if you need me to, whatever.
My kids love it, they have been using it for three years, my son started at age 2 and my daughter at age 4. We have never seen anything inappropriate on the site. It's the only internet we allow with little supervision.