Warning - this is a mini-novel.
I have a 4 year old son who was diagnosed with type 1 diabetes at 24 months. We took him for his first trip to Disney World in the spring and we all had an awesome time! He will need some extra considerations you would never have needed to make in the past, but I promise that won't ruin his or anybody else's good time!
For us, gaining experience in guessing carb values was important for us feeling comfortable since as PPs have mentioned there are no posted carb values. Individually packaged items like Mickey ice cream bars and the items in some of the kids meals (uncrustables, grapes, etc.) will have nutrition labels, though. Calorie King is our Bible of nutrition values and even years into my son's diagnosis I refer back to it all of the time. They have an app for iPhone, but I really prefer the hard copy book and carried that around with me.
My son wears a CGM and that also made us feel much more comfortable, but I know that is not for everybody. It will be important to test blood sugar often especially since he's so young and it will likely take a long time before he is able to recognize when he feels low.
You'll want to be sure to bring treatment carbs with you and always keep them with whoever might be with him. If you're splitting up your party, you won't want him to have a low while waiting in line for a ride while his treatment carbs are with a different family member across the park. I brought little packets of honey and glucose tabs with us since both are lightweight and easier to lug around than something heavy like juice boxes.
Even if you did know what all of the carb values were for everything he eats there, there would be other things that may throw his regular insulin needs out of whack. And everybody's body reacts differently. Heat, adrenaline, excitement, increased activity from all that walking, etc. can cause some crazy things to happen to blood sugar. I have no idea why, but for my son the first day of any vacation we take he tends to run normal to low but any subsequent days he is high until we get back home and into our regular routine. Not just at Disney where we are guessing at most carb values, but even if we're having a relaxing trip to the beach and eating in the entire time where we know the carb values for everything he eats.
Also, pack double the amount of supplies you think you'll need to bring with you just in case.
Feel free to PM me if you have any questions that I might be able to help with. I know it can be overwhelming early into diagnosis.