In your blog, you said you were going to put a roll cage on it...I wouldn't really suggest that.
1. With a roll cage you need seat belts. If you want to fab up some mounts and stick em on, be my guest.
2. this looks like a factory made cart (maybe a clark?) which was not designed for a roll cage.
3. are you a good welder? if you do make the cage, make it god and strong.
I also suggest co-posting on here as well as your blog. That way we can help you fix problems that may arise before it is too late, or offer you some advice to make your build better for your kids.