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