Ok, from my experience there are a few things here that should be addressed. I don't think that they study is wrong but there are some considerations about the situations. First, not surprised that there is the interference issue. I would much prefer to read from my Kindle than my computer screen, we have lots of years reading with a position and focus range that has become comfortable, so with that and the lack of things like email popup or text messages, it makes sense that readers with physical books or devoted ereaders would be better, also while the iPads have a nice size screen, they do have all the other distractors and most phones have a much smaller screen for reading. To me Madeline has the most important quote "I don't mind what I read as long as it's interesting," to her it is the story, and she is fine with Kindles. The part about the pictures is also important and is something that I think that ereader programs should consider. We already have lots of research about how readers do actually pick books by covers (and in many cases thickness), there was even interesting research on young readers picking books by colors (look at the International Children's Digital Library on book choice and you can see the result of that research). So to me a good thing to have as part of the basic ereader program would be that every time you "open" your book to read it, for at least a second or two the book cover would show. I also wouldn't be surprised if there wasn't a social aspect with the physical books, they show their covers out to the world and that would allow others to open conversations or experiences about that book, something that is harder with an ereader. As for children preferring paper, that could also be an experience thing. Parents have been buying paper books for small children, so their "pleasant" experiences about reading have been paper based, so it would be a change to move to digital print. Ask any parent about getting their child to change from one (high experience) restaurant to a different one (even if it serves about the same) and you can hear about the problems of change. At the same time I'm not surprised that many children my prefer reading from paper, it is much more tactile that a screen, although with that you might want to look at the research about germs in books (see ebooks and the sickroom: http://drscavanaugh-ebooknews.blogspot.com/2014/10/the-ebook-and-sickroom.html).
The greater access students have to electronic reading devices, such as iPads and mobile phones, the less likely they are to read, according to a recent study by Murdoch University lecturer Margaret Merga. Findings also show that frequent readers prefer paper books.