We've dabbled in some programming together previously, using Scratch. They seemed somewhat interested, but being typical 6 or 8 year-old boys, their attention drifted and I haven't seen them working with Scratch recently. The Hello World! book uses Python, which is very different from the graphics-oriented Scratch, so I have no idea how if their interest in programming will come back. We'll see.
(Not that I necessarily want my children to follow in my computer-programming footsteps, but I think that learning basic programming concepts is important in the same way as learning basic math concepts.)
Anyhow, I ordered the book from Manning.com with the 40% discount code you can find on the Hanselminutes podcast page. It's good until January 31st.