We took DS8 when he was 2 and I thought he was easier than DS11 who was almost 6 at the time and soooo excited.
I suggest bringing something comforting from home like a special blanket or stuffed animal to help her sleep in the hotel.
Bring portable snacks and juice boxes or water- little children seem to be hungry at the most inconvenient times and if you can just pull something out of the diaper bag you will save valuable time.
Also, either bring all your supplies (diapers, snacks, etc...) from home or have them delivered from gardengrocer.com. Supplies are very expensive in Disney.
Bring your own stroller. You can always rent one for the other girls but DD2 (and maybe DD5) may need one outside of the parks as well. (It's an awful long walk from the bus to the hotel room if you are lugging a sleeping child.) Concentrate on what DD8 and DD5 want to see. Chances are DD2 will be happy just being with her family.
Don't be surprised if she doesn't like the characters - you never know with kids that age.
Definitely stay on site. That way if you need to split up and take DD2 back to the hotel for a break, everyone's fun won't be spoiled. See what type of hotel your budget can handle: value, moderate or deluxe and then go from there. They are all great in their own way.
Have a great trip.
