This isn't an easy question to answer, because there are a lot of little things that need to be considered.
My father bought an iPad, much to my surprise. It has become his primary machine for doing pretty much everything. The only time he goes to the computer is when he needs to edit and print out a picture. That's it. He rarely uses his desktop anymore. He didn't even need a keyboard with it. He does all his typing right on the screen. He uses it constantly.
Now, the issue is, my father is a consumer. He consumes information, he doesn't generate it. He does write a lot of Op/Ed pieces for the local newspaper and a lot of email, but that's it.
If one is going to use an iPad for production—writing, editing, images, graphics, etc.— then I would recommend getting at least a bluetooth keyboard. A bluetooth mouse or trackpad would be an optional add-on. Apple does have Pages for iPad, and Pixelmator (Photoshop alternative) is now available for iPads and iPhones. You really can get a lot done on an iPad, but I would recommend the top-of-the-line model with maximum memory if that is going to be what you are doing.
But for actual production work, especially if your kids will be learning computer programming, then you absolutely want a MacBook. It has a lot more flexibility and horsepower for doing the same work I mentioned above and more.
The Chromebook Pixel will cost you $300 less than the MacBook Air, making it the better choice where cost is concerned. But I consider the Apple the better choice for value and quality.
If your school is using the Chromebook as its default platform, it might be best to stick with that to allow better continuity. In contrast, I used to work in Research & Development and pretty much everyone was using a MacBook Pro to do their programming work, myself included. The Apples allowed for a great deal of flexibility, and the Unix underpinnings gave us the same environment that our high powered computing systems were running. I used Parallels to run other operating systems such as Windows or various flavors of Linux when necessary.