The link that Sad gave to Disney with Type 1 Diabetes is an excellent place to start. The folks who wrote that are spot on with many good tips.
A few questions:
When is your trip?
How long has it been since your child's diagnosis?
I think if your child has been dx'd for awhile then it is a much different situation than if you are newly dx'd. Also if you are going during the hot months (summer especially) then you might find the heat can really do a number on your type 1 child - causing unexpected lows (or highs) - we always have more trouble in the heat one way or the other. For cooler months that isn't so bad. All of the walking can lead to unexpected lows too - we tend to stroller William (he is 6 now) between the attractions just to keep some of the exercise factor out of it if the heat is bad.
Post #3 of the FAQ has some links to old threads about diabetes which you might find helpful. Also review the information for the Guest Assistance card (GAC). A newly dx'd preschool child in the heat of the summer - especially if you are on NPH - might find this to be a great deal of help. If you have questions about the GAC and type 1 I'm happy to answer them directly if you can PM or email me.
Another very useful tip is a good touring plan unless you vacation so often to WDW you already know it like the back of your hand.
I really like Tour Guide Mike - it is a site you pay to use - cost is similar to a guide book. His advice is very helpful in avoiding the long lines and heat of the day, etc. If you go to the Theme Park board here which he sponsors there is a link there to a discount for him.
I also
HIGHLY recommend the Children With Diabetes website - chat rooms and forums there are top notch:
http://www.childrenwithdiabetes.com They hold an annual family conference for type 1 kids and their families every July in Orlando - every other year they hold it at Coronado Springs Resort at WDW. Here is a link to their upcoming conferences:
http://www.childrenwithdiabetes.com/activities/ People on the forums also discuss doing Disney with type 1.
My younger son William is 6 yrs old now - dx'd when he was 26 months old and started pumping at 32 months old with Animas. We've been to WDW lots of times and on a
Disney cruise once since then. I'm happy to answer any questions.