Does your 8 year old often break and mistreat his things, or do you say that just because he's an 8 year old? I can almost guarantee that if I had a Kindle at 8, I'd probably still have it today*. If he breaks and mistreats his things, maybe he shouldn't have an ereader at all? It's fate is almost assured. Just food for thought.
In your shoes, I would probably go for a K3 w/ wifi with ads and put it in the Amazon stock leather case. I wouldn't bother with refurb, it's like $15 difference. The stock leather case is really good and very strong. It's also the only case that I know of that actually holds the K3 by the chassis, not by weak little straps at the corners. If he drops the K3 and the case is on, even if its open, the K3 will probably be okay. it won't bounce out or skitter away like I've seen happen to people who don't have it.
Everyone drops their device at some point during its life, and they are meant to take a bit of it.
Another thought is to look on Ebay for a used Sony 505. These are metal and seem to be built like tanks. I've dropped mine on many occasions, works fine. A slight marring on the metal and that's about it.
*Yep, I know I said if I had a Kindle at age 8 it would still be around today, and then admitted to dropping the 505 on many occasions. It's still around though! lol