But this is a bit of an unrealistic scenario you've set up. It implies that the child has no exposure to Star Wars until one day, when you sit them down and they suddenly have the patience to watch a 6-movie marathon. Even if you break it up across 6 days, you're assuming that they don't know anything about these characters and their origins yet. In my house, that's just not possible.
With my oldest son (now 5) I never really had a plan to indoctrinate him into Star Wars, and so he's just picked up bits of movies along the way. The first one he actually sat all the way through was ESB when he was 4. (Which speaks volumes to me about how a great story doesn't need "kiddie" elements to appeal to a child). Now his two favorites are the clone wars cartoons (the best part of the prequels if you ask me) and ROTS. But in my household, ANH (formerly Dr M's version, currently Adywan's Revisited) gets the most play.
(My youngest son, now 2 years old, is fascinated by Darth Vader. He does the trademark breathing and sings Darth Vader's theme. He even asks to watch "sawas".)