Well-read parents are notorious for foisting classic literature on their kids at an early age, whether those kids are really ready for it or not. Not only can the themes be difficult, the words themselves are often incomprehensible, and referencing anything unfamiliar in a dictionary isn't usually high on a young person's list of priorities.
Fortunately, the Educator's Classic Library produced a set of books in the late 1960s (sadly, they're out of print) that paired classic novels with explanatory annotations in a kid-friendly format. Each book is tall, hardcover, and uniquely illustrated, with sidebar notes defining terms, describing flora and fauna, and when appropriate illustrating objects.
Most of the texts are complete and unabridged, so kids get the full experience of reading the book without the headache of being lost half the time. An afterword at the end of every volume (called a "Backword") offers further context for the story, as well as biographical information about the author and notes on his cultural and social environment.
Don't let the somewhat old-fashioned covers fool you: these are fantastic books that are more likely to pique an elementary or middle school student's interest in classic literature than a more bland text-only version. At the same time, you don't have to sacrifice the beauty of the original work, or worry about gaps and holes the author never intended to leave.