This year I’ve been spending quite a bit of time teaching my nieces and nephews about programming, as well as facilitating a Girls Who Code chapter. These are some of the resources I like for teaching kids how to code:
Made with Code is a pretty cool website with small exercises and projects that use Google’s Blockly and are designed to encourage girls to code. It’s definitely pretty limited content-wise and I found some of the projects to be borderline painfully gender-stereotyped, but it’s free, reasonably well implemented, and does a good job of providing exercises that teach kids to start thinking algorithmically.
Some other popular resources are websites like Codesters (free, pretty good, but a bit buggy and still only uses Python 2) and Khan Academy (free, OK, but their Classroom resources are pretty poorly implemented and I’ve found bugs and inaccuracies in several of their tutorials).