Resist the urge for an abridged version. You can't abridge this story. It's all important. Is it long? Yes. My Kindle informs me that it will take me about 38 hours to read to my kids. So what? That's just about six weeks. We're going to be reading something - why not this? And guess what? Monte Cristo is a book you can read over and over again. It doesn't get old. In fact, there are scenes that I read over and over again. It's like The Myth of the Grand Inquisitor in The Brothers Karamazov. You don't have to read the whole book to pick up it up in the middle and read one part. (If you have not read that book, stop reading this post and go read that book. Now.) Read The Grand Inquisitor part many times. I have become very disillusioned with my undergraduate professor who made me fall in love with political theory and inspired/consigned me to a PhD in Political Science (long, sad story), but I will always love him for introducing me to the Russians.
I have gone far afield. See what wonderful things happen when you make the classics your friends? Introduce your kids to these friends early. Don't worry that they won't understand every part. It doesn't matter. They'll understand something far more important: these books are not intimidating. They are not out of reach. They are friends to be revisited many times with new things to be learned from them each time. If you are worried about a particular word or scene, edit it as you read. They will never know! Well, okay, your 10 year-old may not be read for Tess of the D'Urbervilles, but there are plenty that she is ready for. Rediscover some old friends for yourself or, better yet, make some new ones with your kids.