My son (10) got a nook simple touch for Christmas this year. We did look at the Nook Color/Tablet but decided that a dedicated book reading device was a better fit for his age. Yea the Tablets are nice but they are also full of distractions that we felt would be too tempting for him to ignore when he needs to be reading books not playing games/surfing the web/watching moves/ect..
As far as kid books go our only requirement, that he agreed to, was that he had to get his AR (Advanced Reader) books read first before he could read just for fun. It should be noted though that most everything he wants to read is on the
AR book list so this should not be a problem. Anyways back to the books so far pretty much every thing he has been reading this year has been available as an E-Book so I'm not too concerned about finding content for him.