As for other "kids" friendly environments, you have + the MIT Scratch/ Blockly drag-and-drop visual coding environment (Kano has blockly and they build the interface with the RPi Minecraft (cf https://www.youtube.com/watch?v=cTjN_1GFsfk ))
+ Python / Ruby / JS "for kids" books; HumbleBundle is currently having a book bundle that include a few "Programming Language" for kids book https://www.humblebundle.com/books ; you will find online a few other books (e.g. https://inventwithpython.com/ http://ruby4kids.com/ruby4kids ) based on doing games)
+ you can have the Sugar image (what's the OLPC is running) setup to run , http://wiki.sugarlabs.org/go/Welcome_to_the_Sugar_Labs_wiki . It has a lot of nice apps, and it includes a nice Logo inspired Turtle environment (TurtleArt / TurtleBlocks)
If you decide to go into hardware as a way to get your kids started, then look at Arduino also (e.g. http://sylviashow.com/episodes/s1/e3/full/arduino ), or a simple robot controlled by your RPi.