we took our then 4 year old son and 9mth old baby with us for the first time last May - It was FANTASTIC!!!

We were there for over 3 weeks and did everything we wanted to do - All the disney parks (inc water parks), Seaworld and Aquatica, Universal and Islands of Adventure - the last two (UO parks) were meant to be for the adults and we didn't expect our son to like it much - well IOA was easily his favourite place!!
You just need to be clear on what their limitations will be before you go and don't worry about the things you can't do, and concentrate on the things you can - We didn't expect our DS4 to go on any of the bigger rides (it turned out we were wrong and he even loved Tower of Terror


), but he asked to go on those, we hadn't even planned to look at anything like that.
The firm favourites were the water areas, play parks, water parks and the little kiddie rides, but we knew that would be the case and just had the best time watching them at that age. Our youngest LOVED the characters and that was a thrill for us all by itself!!
The only thing that was on the downside for us, was our eldest really struggled with the heat - so your plan for December sounds perfect!! (In fact our next trip is now going to be Dec 2010!!

)
If you have any specific questions let me know!! - happy planning!!

