I've been teaching my kids how to code on and off since they were six. At a young age, it's better to think of it as creating animations using Scratch or similar. As they get older they can transition from making animations to making games.

A good way to start is with code.org.

Coding is much more fun if you can share your work with others. If the kid can bring an iPad to a friend's house, great, use Hopscotch. If you want to email a link to your game to Grandma, you should probably use Scratch but until Scratch 3 comes out, Grandma will have to have Flash installed.

If you have Android devices lying around, try MIT App Inventor. As your kids get older, you'll want to transition them to "real" coding with text rather than blocks. Check out Greenfoot and Phaser.

On each of these pages, you'll see more of my thoughts on various options:

On August 28, the GNOME Foundation announced the 2019 Coding Education Challenge, a competition aimed to attract projects that offer educators and students new and innovative ideas to teach coding with free and open source software.