Windows/Mac: If you're looking for a kid-friendly way to let a young child interact with your computer without the risk of them deleting important files or getting into content they shouldn't see, Zoodles is a kid-friendly browser.
Older children can—and should!—be taught to use a computer as it is and not in a walled garden but younger children certainly shouldn't be let loose on a computer. Zoodles is a free browser for Windows and Mac machines intended for children 2-8 years old. One of the most interesting features of Zoodles is the interface scaling. Small children can use Zoodles with a text-free and simplified interface and as they get older and acquire reading skills you can increase the complexity of the interface to include written text and more detailed GUI elements.
The basic version of Zoodles is free, upgrading to the $7.95 a month premium version adds in additional functionality like parent reports, advanced filtering, and the ability to filter out games and content beyond what the kid-friendly filtering included in the basic version provides.