You might want to try a bit of a different schedule for the 6 year old. Teens on caffiene can go all day and all day, the 6 year old probably needs a mid day break for a swim and a nap. Heck, I'm 40 something and _I_ need the midday break!
I would consider allowing the teens some time on their own at parks, and meet back at a designated time/place. It's NOT Disney, but one year we took nieces and nephew to 2 theme parks. The older kids were 12, 14, 16, the younger was 5. What they wanted to do was totally different, so hubby and I split up and one of us took the older oens and another the younger one. (These parks were not as safe as Disney, so I wasn't comfortable turnign eth teens loose.)
Rise your teens may love might scare the begeebers out of the younger one. Be sure to ask about SL interpretation services as well. Call early and ask for Guest Services.