Please reccomend best kids development environment

My 6 yr old spent hours yesterday starting to compose the storyline and interface for his first digital game. He's using Paper App on iPad to sketch it out. I am amazed at level of sophistication he is showing having designed a lit of interfaces for apps.

Any of you have recommendations for development platforms for kids that will teach him fundamentals, while still letting him create something to be proud of? I want him to be able to see results, while still challenging him to learn. Thanks for any help you can provide. I have a tiger by the tail.