I've thought about this one quite a bit over the past few months. Teens have their smartphones, many of which can be used with the Kindle software, but what about younger readers?
My instinct (and some very cursory market research) says no. Most parents are unwilling to place a dedicated $150-$200 e-reading device into the hands of their children, not when a $10-$15 paperback is almost bulletproof by comparison. (I don't mean all kids are careless or clumsy, but accidents happen and the humble school bag tends to be a concrete mixer and compactor all in one.)
I think Pottermore will change this up a bit, but it depends how much the HP ebooks sell for. (I've heard people saying 'who's going to buy the ebook when they already have a print copy?' ... um, do you know how many NEW kids there are each year? It's an endless market.)
But that's beside the point. There's one very good reason to offer middle-grade books in cheap ebook formats: parents. They can read a preview (or buy the whole ebook), and order the paperback if they think it's suitable for their kids or grandkids. Think of it as e-browsing.
There's another reason too: Many adults enjoy teen or middle-grade fiction, but wouldn't be seen dead reading them in public. Stick them on the Kindle, and who's to know?
Simon Haynes is the author of the Hal Spacejock and Hal Junior series (Amazon / Smashwords / other formats)