You head to Guest Relations-in Magic Kingdom this is in city hall, to the left just past the entrance. In Hollywood Studios it is also to the left just past the entrance. In Epcot it is just under the big Epcot globe. In Animal Kingdom it is to the left after you enter, in the same building as the bathrooms. All guest relations have big blue letter "i" on them.
You don't need to make any prior arrangments, actually this isn't even possible. Bring a letter from you doctor. All it needs to say is that your child is a Cf patient, has issues with heat and standing long periods of time, please allow this family to not wait on lines. You will probably not even need the letter, but in all the times I've gone its been handy a couple times, just in case you get a cranky cast member.They will give you a guest assistance card.
It is not a NO LINE card. It will take you to a special entrance at each attraction, or through the fast pass entrance. It will reduce your wait time significantly-at times you won't have a wait. I only mention this bc when they give you the guest assistance card, they clearly tell you this is not a 'no line' card. So don't be surprised. No one has a no line card, not even the most of disabled people. However, Disney does their absolute best, especially with kids so this card will really help you.
With me and my kids its really helpful, bc we can do the parks in about half the time, so I can get back to the hotel mid day for my treatment.
You can also bring your child's nebs, vest, meds, whatever to first aid, leave them their, and do therapy there if you need to.
Get the disney fan spray bottle when you get there. Its like $12, but its battery powered and keeps them cool and works alot better than walmart ones.
My other tip is take breaks. Don't feel like you need to go on every ride. Lots of dining options are outside-Find the airconditioned indoor restaurants and make some type of plan to have breaks there. (if you want to know where some of them are in each park, just let me know) And eat good-half the fun of Disney is the FOOD!
If you stay on Disney property, when you book your hotel (or if you already did, call back and tell them) you are a special needs family and you're celebrating a milestone..reunion, acheivement, whatever. They will give your kids a cool mickey pin recognizing that and a few other treats. If they wear the pin through the park, cast members will notice it and give your kids little treats or special greetings.
Enjoy!