Consider something simple and basic
Since the two boys are 7 and 8 years old, and you want them to be reading instead of playing games, I'd suggest one of the more basic black-and-white Kindles or Nooks. They don't need color graphics or a way to access e-mail or the Web; keep them focused on using the device to read.
I know some libraries lend Kindle-format books (I have a Kindle Fire), and I'm pretty sure the Nook format is available at libraries too. Just what the public library offers where your nephews live, you'll want to find out before you make a purchase.