You should get a lot of responses to your question, because not all families are made of 8, 6, 4, 2 year olds.
Is the 16yo mature? Would you let her do somethings on her own (with cell phone in tow)? Or what about DH going with her on the teen rides (RRC, TOT, SM, SplashM, BTM, EE). You may want to split up for a section of the day to dedicate to her needs.
Or, give her a day or two that she can plan - so she feels that she has control. If the little ones don't like it - bear with it. I'm sure, there's lots of times people have done it for them.
Accomodations is based upon wallet, no? Your af family of 6, so either a DVC or 2 rooms. Depending on your expense budget, there are loads of options. A 2-bedroom unit at Boardwalk is a nice, well rounded option. It has a work-out room, game room and entertainment on the boardwalk for her and her siblings. If she's into sports, ESPN. Spoodles is a great place to get pizza.
I love character breakfasts and think any age is a great age for them. Is she into princesses? CRT
Is she into Mary Poppins, Alice in Wonderland (when she was younger?) - she may like 1900 Park Fare.
World Showcase seems to be a great place for teens and so is MGM (due to the rides, I believe).
How about taking her parasailing at Contemporary?
How about taking her on a Segway Around the World (at EPCOT - before the WS opens)?
How about renting mice at GF with DH or yourself and have fun at the lagoon?
How about swimming with the Dolphins in LIving Seas?
How about Keys to the World tour at MK?
West End at DTD is cooler for the hip teen - have her spend time in Virgin Records, while the others are in
Legoland or where ever.
I think there is lots to do, if you take advantage of the WDW options.