A Kindle would be great
I was going to reccomend a kindle fire hd 7 inch, but you're right, if it's just for reading, a regular kindle would be best. If they will be reading in the dark, I would reccomend spending a little extra money for a kindle paperwhite. But of course, if you get them that, you may not get them to stop reading at night! Then again, that may not be a bad thing! Good luck!