I know I'm late to the thread, but we used the Sit n Stand that Pooh0200 had a link to, from Target. We had our older two in it, as our youngest wasn't born yet, and it worked awesome. The only drawback I can see with a baby, is if the baby is no longer in an infant carrier, that front seat does not fully recline so the baby can comfortably sleep.
But with my older kids, my oldest child, dd, loved being able to stand in the back and ride. It made it a lot easier to keep track of her, too, in crowded areas, such as through Adventureland.
Given that your dd(6) doesn't want to try the Sit n Stand, I would probably just rent one. It will be less cumbersome, you have extra grownups to help with the pushing, and if she decides she doesn't like it, well, don't rent it the next day. And you wouldn't have carted a huge double stroller to DL for no good reason at all.
Good luck, and have tons of fun for me!