I am working through 'the Hello World book'with my kids and plan to teach an after school course on Python next year. I am very pleased with 'Hello World' but am curious on your thoughts on which one you prefer. 'Hello World' or 'Python for Kids'?
Python for Kids is really good also, has a intro about the Python language which feels more focused than the other book and it has a chapter about drawing with a turtle, as I did with LOGO many years ago! ... Python for Kids also has plenty of notes about running it on Windows, Mac OS X and Ubuntu so you'd be fine on any OS you have.
I think you should try both, but the best order would be to finish Hello World first and Python For Kids second.