In an ideal world, a Kindle (or Nook) for each of your kids, and an iPad for you.
The Kindle does text to speech, so your ds could listen and follow along. (Also, if you already own the actual audiobook, you could put it on the kindle in mp3 format and he could listen while reading.)
Another benefit for your family is that you would only have to buy a book once, and then all your dc could read it at the same time on their respective Kindles. And with the Kindle app (or B & N app, if you decide to go with the Nook), you could read the book on an iPad as well.
I would not get your kids iPads for reading, especially if they read a lot. Kindles/Nooks are much easier on the eyes because they are not backlit. E-readers are smaller and lighter. Also, there is the temptation to play games, watch videos, use internet with the iPad, rather than reading.
For you, I think an iPad would be worthwhile. Lesson plans, e-books, internet, other useful apps - I think you'd get a lot of use out of it.