Treasure Island. Unlike other Classics Children Ought to Read -- Robinson Crusoe (written before the idea of a novel as a coherent narrative), Swiss Family Robinson (written for moral edification) -- this is actually a really good book!
DD6 was intensely interested, not at all put off by the difficulty level (though with lots of explanations from me, both of vocabulary and historical context). But she got pretty freaked out by how scary it was. She had been asking for books with "wicked" characters, but this was a little too rich for her blood. Unlike their rather sanitized Disney descendants, these pirates are freaky nasty scary.
DD's evaluation: "Let's read this when I'm older." May work for some in this age group, maybe even DD by the time she's 8.
(P.S. As a less scary substitute for her blood-thirstiness, I've been reading her the Tintin adventures The Secret of the Unicorn and Red Rackham's Treasure.)
