Jean,
I'm a planner too so dont worry about the number of questions, I get it.
We didn't have any major meltdowns. I will admit it was almost a sensory overload in the first 2 days for the 3 yr. old. But we talked to him about Disney World for about 45 days before we left to explain what we would see, who we would see, the rides, etc. and I think that helped alot. What I did was make a paper chain (an idea i got from Deb's site i think) and we would remove a link every night to show him how much longer before we would leave. Also, I bought the Birnbaums WDW guide book for kids, by kids and read him sections out of it at bedtime to help him prepare for the rides, etc. While in the parks, etc. I just made sure my son knew that I expected the same behavior out of him in WDW as I would anywhere else or at home. Once he realized that, he was fine. He realized that he couldn't get away with anything more than usual and he could still get in trouble there just like anywhere else.
Both did great in the restaurants, mostly we ate character meals in restaurants. Other meals were eaten in the room or a hot dog on a bench.
My children LOVED the characters up close. If you have any place near your home where there are any kind of characters dressed up, take them there to see how they will react. We did a Christmas thing last year where they had a reindeer, snowman, and some other characters dressed up. My son loved them and that didn't change in the World. It was a good indicator. Some kids are afraid and some just aren't.
My parents took my son to Fantasmic twice. I did not see it. I was worried about my son seeing it because he is very afraid of the dark and "bad guys" but he did great. He actually loved it because the "good guys" won. To help my son on dark rides (and we did do Pirates), I bought him a small not so bright flashlight and that gave him some sense of security. Also, if he got scared, one of the grown ups would put him in their lap and he would be okay. He even did splash mountain and big thunder mtn. railroad - i couldn't believe it.
Hope thats everything. If I forgot something just ask.
wendy