Remember at WDW after you've parked the car you will still be 30 minutes away from the park. So add that in to your timings. To avoid the crush get to the park at 2.30pm in time for the 3pm parade and get a place by the entrance. After the 3pm parade the park empties considerably and you can then stay till late.
Bear in mind that it's complete hell trying to get out of the park after the 10pm fireworks. Stay for the electrical parade which is wonderful but watch the fireworks from the carpark as we waited nearly an hour just to get out at 10pm with 2 v. tired kids.
For a first visit I would recommend WDW, Sea World, Animal Kingdom and Busch Gardens (Tampa). If your kids are older take them to Universal Studios, MGM and Epcot.
Don't try and do too much, it's too hot and exhausing and you'll come home needing a holiday.
Definitely go for International Drive or Lake Buena Vista for accommodation. Kissimmee is a long way out (no matter what the brochure says) and they have had roadworks on the 192 for over 2 years now and the traffic is appalling.
There's a great Super Walmart on the 192 though! Its by Wild Bills dinner show. And it's 24 hours!
R.