Dickens is in my list of top 5 favorite authors, so please don’t see this as a criticism of his work. Here’s my opinion: Yes, a 6th grader can read Dickens, but only if they have the wherewithal to see it through. The books are quite wordy, being filled with many long, drawn-out descriptions. If you’ve ever read Hemingway, I’d say Dickens is pretty much his opposite, as far as writing style. I have started my two oldest with Great Expectations as their first Dickens experience. I think it is an easier read than his other books. My personal favorite is David Copperfield. The storyline is easy to follow, but the book is quite long. A Christmas Carol is not very long and your kids may already be familiar with the story. For my kids, however, I thought the “spirits” would bother them too much.
By the way, I wouldn’t discourage kids from reading long books. It’s just that with Dickens you can kind of get bogged down sometimes. You have to be willing to keep reading. His books are always worth it.