E-readers for children: Nook HD vs Kindle Fire

I would like to buy an e-reader for my 7 and 10 years old children.
I would like to buy something in colour, so they can enjoy the pictures as well as the story.
I also need something that is only for books, and not another avenue for playing video games (so this rules out tablets).
The device needs to be sturdy, because children are destructive, no matter how well behaved.
At the moment I cannot decide between the Nook HD and the Kindle Fire.
Which one would suit my children better? Are there other alternatives?
