Definitely read this website: Disney with Type 1 Diabetes:
http://allears.net/pl/diabetes.htm
It is written by the parents of two type 1 kids and covers most of the bases. Their children were old than yours when dx'd so you will face some challenges that they might not cover - but they cover most.
My son was dx'd when he was 26 months and is almost 7 now. He also has a couple of life threatening food allergies (peanuts/shellfish) which make eating a challenge (dining out that is). We first took him to WDW after diagnosis when he was 3 yrs. old and we go back often.
My son is almost 7, as I said, and still doesn't often recognize his highs and lows. By the time your son is that age maybe he will - maybe he won't. But don't feel bad if *you* cannot tell either - it is a very subtle thing with many kids. And when they are excited (like at WDW) - they often do not want to stop to let you know they feel bad.
You will almost certainly have issues at WDW that you do not face at home on a routine basis. You are going when the weather is mild so that is in your favor. The heat has seemed to be the hardest thing for my son to deal with. Walking/exercise is only recently an issue (he was also hypotonic at birth and didn't walk until he was about 2 - and was not walking the parks until he was 5-6 or so). I highly suggest using a stroller for when your child wants to ride. Maybe he isn't a stroller kid but it will save some energy and perhaps prevent lows from all the exercise.
Your one year old probably won't care if he/she rides something or not but the 5 year old will (and your 4 year old). I suggest splitting up at times if your child with diabetes is having an issue.
You do NOT need to go in the bathroom to handle anything - testing, shots, whatever. Take all that stuff with you and do it where you are. If you prefer privacy - that is fine - but there is no need to do it because you worry about what people will think. We test and treat in public. I have changed his infusion set at the park and I do that at First Aid in a private room. First Aid is awesome - very helpful.
Many people here who will offer advice - are much appreciated - but also NO ONE BUT YOU knows your circumstances, issues, limitations, etc. So when it comes to getting a GAC, do what you think you need to do. You can always get one after a day or two if things are going poorly. Diabetes in a 4 year old vs. diabetes in a 10 year old or an adult is sometimes two different things all together. I find the time that we have *really* needed it was when it was very hot out (July hot). If you ever get to go to the Children With Diabetes Friends for Life conference (held in July 2009 at Coronado Springs - FYI), you may find that they have cast members on site to give families a GAC. There are guidelines issued by the conference for using it. Nothing wrong with knowing what a GAC is - whether you need it depends on your unique situation. I've said enough about that in previous posts. Anyone who tells you that you won't need a GAC does not understand that the GAC is issued for a person's issues and not for a diagnosis.
What I recommend more than anything is to go with a good touring plan. If you are going during spring break (I sincerely hope not) - you will need a good touring plan and a choir of angels on your side...heh! But seriously, we use Tour Guide Mike - and I know it is hard to follow his advice with little kids but his plan is flexible and you will NOT be waiting in any lines. You will be fast passing most things. His touring plans are without equal - IMHO.
And finally, with young kids and one with type 1 - plan now that you will *not* see it all, do it all, eat it all, experience it all, etc. Lowered expectations have served me very well - especially when my kids were young. Sometimes it is just fun to hang out and let them play in one of the interactive Epcot areas, or at the splash fountains, etc.
I wish you all the best! Feel free to email me if you have other questions.