Nobody does vacation fun like Disney.
My wife and I took all 8 of our kids twice and stayed for a week the first time and two weeks the second time . . . and that wasn't enough. The special meal plans you can get if you stay at a Disney resort are great. A friend told me about his experience (meal plan) with his family and they loved it. (He didn't tell me until I had returned.) If you can arrange to participate in a character breakfest, do so. To save money on meals, if you don't get Disneys meal plan, we bought buckets of chicken for dinner, two liter bottles of soda and potato chips and tortilla chips from one of the grocery stores and ate at our hotel around the pool or in our room. Sam's has an eating area and you can buy rotisserie chicken there or already cooked pizza much cheaper than anywhere else. Walmart has the least expensive souviniers, including t-shirts. Go to the parks the first day. Then go to Walmart for the souviniers, and go back to the parks wearing your Disney t-shirts, etc. the next day(s). (If you REALLY want to go cheap, e-mail me.) When our kids were older, we would pair up the oldest one with the youngest one, the second oldest one with the second youngest one, etc. for the morning. We'd meet for lunch ($.99 hamburgers from McD's, Burger King, Wendy's, etc and cans of soda from the grocery store) and then let our kids go off, in pairs of their choice on their own and we had the littlest ones. They had a wonderful time. We had a wonderful time too. Then we'd meet for dinner. And we'd meet to go home. Some of the kids chose to stay with us some of the time and dad would buy the ice cream or what ever as we visited the attractions.
If you are into gardening, Epcot had a behind-the-scenes garden tour that is great, but the number of openings are limited.
The last time we went we bought the 10-day park hopper that included the water parks, but the water parks weren't counted as part of the 10 days. Typhoon Lagoon will be wonderful for your older two. They will have a blast. 10 days was not enough and we had to choose which attractions to do and which to pass up. :(
Disney has a cruise line. You can get a 7-day deal and spend 3 days on a cruise and 4 days at the parks, or 4 days on the cruise and 3 days at the parks. (They also have a 7 day cruise.) The only bad part of that will be knowing how much you missed at the Disney World parks. You'll want to schedule another trip. This is a wonderful time to schedule your trip because the tourism industry is hurting and the discounts are fantastic.
If you've never been on a cruise and you have enough time, a Disney Cruise is worth your time. They have a children's program that is the envy of all others. (Princess Cruise Lines also has a fantastic kids program. Our kids couldn't wait to finish breakfest to go to their kids program, and hated to leave when we picked them up for dinner.)
Universal Studios had the best Luau entertainment of anywhere we've been, including the three we went to in Hawaii. Hawaii wins on ambeince, but Universal wins on entertainment. (There is a "Henry the VIII" dinner threatre in Orlando, but it was worse than awful. Not a Univeral Studios program. Hopefully its no longer in business.)
I could write more, but books have already been written. Good Luck.