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would look at the Kano Computer. It’s a $150 Raspberry Pi based computer that your kid puts together. It uses a Raspberry Pi as its starting point, and is meant to teach kids about the nuts and bolts of Computers and the basics of coding. It has an open source, Linux based OS that is designed for kids to use and lets them modify games. There is also a tie in with Codecademy, so it can be the springboard for more traditional educational outlets as well. I know that I sound like I work for Kano, but I don’t. I thought Kano was cool when they made their kickstarter pitch and I think it is cool now that you can actually get it. http://www.kano.me/
I think it's awesome that you want to get your kids involved because IT is the way up! While I personally cant stand programing, I enjoy computers more on the networking side. However, I ran across a few books while at the library that were driven towards teaching kids about programming. To my knowledge one of the best starting programing languages is python, and seeing as how it falls under GNU it's free to use. Below is a link to a book that may interest you, and I am sure that you would be able to find possible PDF formats or even free versions.
Hopefully your kids pick up on it and enjoy it.