I think the comments reflecting on how we all have differing opinions as to how we do WDW are spot on. As Wayne said there is no right or wrong way to do it. From how you tell your kids, to the planning aspect all the way through what works for one family would make a miserable holiday for another.
And from my point of view I think that 2 days is more than enough for kids of the ages we are talking about to get involved in the planning, especially when they have been 3 times before and you know exactly what it is that will make it a holiday to remember.
Louise you know your kids better than anyone and I know that when you spring your surprise they will be jumping up and down for joy.
Just don't let Dan be holding a chocolate milkshake when you do tell them though will you