To me, the beauty of ebooks, and ebook readers is that they are just text. No pictures, no interactivity, no annoying videos with cheesey American voice-overs. By being just static text, ebook reader batteries last weeks before needing to be charged. By using e-ink, the screens can be read in blazing sun. Neither of these massive advantages can be matched by tablets, whatever the OS.
The physical books in my house range from "first words" to adult fiction (not that kind of adult literature, get your minds out of the gutter), and the thing you notice about them is that, as you get older, pictures get smaller and words become more important, until the books you read are just text. If a book is well-written, pictures aren't needed, as the world you are reading about will be created within your own imagination.
Some may say that picture books can be viewed on tablets, true they can. But, cramming beautifully drawn pictures onto a poky screen reduces their impact. There are some childrens books that contain artwork that deserves to be appreciated in full, spread across both pages, either of which is bigger than a tablet screen.
There are some instances where interactivity is useful, technical and reference books for example. There are definitely some situations where 30 seconds of video could more than replace 3 pages of step1... step 2... etc.
Children should be encouraged to read real books, to develop their own imaginations. Watching a video is easy, creating a whole world in your mind takes some brain power.
So, thanks, but I'll stick to plain text on a dedicated ereader, and enjoy the fresh air and sunshine, whilst tablet readers watch their videos indoors, before running off to find the charger and an electrical socket.