Hi Gills
Your older ones will love the whole experience, although you will be in trouble for not having taken them before! Yours probably are doing the too old and sophisticated routine at the moment, but the second they hit WDW they'll realise it's all families of all ages and will really enjoy it. Prices do change and there's always a better deal after you've paid, I'm afraid.
Make sure you do the evening shows like Fantasmic, Illuminations and of course the fireworks at MK. Plan for some sit-down meals as it gives you the family time you don't often get at home (mine are 13 and 15 so I know!) Places like Marrakesh in Epcot might be appreciated if yours are male, and you can always blame it on Dad wanting to see the belly dancer. The buffets are also great for filling teenagers up. The roller coaster lovers will enjoy Universal, especially Island of Adventure, but they do seem more theme parks than Disney (you'll understand what I mean when you go - more rides, less magic). The water parks are great for all ages. It's also worth taking in the shopping as prices are so much cheaper than over here.
Enjoy!!!!