My daughter has used (since she was 4) Windows, Linux and Macs. She never even asked about the differences; she just adapted straight away.
You don't want your kids growing up on OSX and never learn windows only to go out in the corporate world and have to work on a PC, they would be at a disadvantage. Until the 95% windows market share changes i think its important for every kid to learn windows first then discover the joys of Mac later as many of us here have done.
Why get an iBook, anyway? The macbook's are much smaller and lighter and easier to carry. You should get a macbook if your getting a mac laptop.
By the time I was in grade 8, I was programming on the Apple ][ in my room and at school, working with DOS 3.3 on the 386 PC in my dad's office, and had used a Macintosh SE as well as a VT100 terminal connected via modem to dad's office. I was also actively dialing into BBS's and quickly figured out the interfaces on all those (mostly Maximus). Never once did I ever struggle to figure out which was which.
My grade 7 science fair was especially interesting. My project was the venerable "which brand of battery lasts longest?" (Duracell, it turns out). I wanted to show a set of voltage/time graphs. Today, you'd plot the data in Excel, make graphs, pop them into PowerPoint, run the whole thing on a laptop. Back then, I had none of that... so I wrote an "etch a sketch" program on the Apple ][, drew all my graphs in that, saved them as raw memory dumps of the graphics buffer, and then wrote the slide-show program to display them all!
Kids are way smarter than we think to give them credit for.