I realize how difficult it must be to make a decision like this, based on what may only possibly happen. I also think the doctor in this case is not being as cautious as he should be. It is not a good idea to travel on a plane if you have an illness that is spread through airborne particles, the way chicken pox is spread. The ventillation on planes is a closed system, so the old air is recirculated throughout the plane. They do have the ability to add in some fresh air from the outside, but on many or most flights, this is not done in sufficient amounts. So the same old air, containing all those germs keeps going around and around.
Also, keep in mind that the chicken pox virus is responsible for shingles, which is an adult disease. This can be VERY serious in many adults, and is also extremely painful. A friend of mine came very close to being blinded by shingles while in his 30s, and in very good health. Many older people contract debilitating cases of shingles, which can linger or recur. This virus is not one to mess around with....
I would try calling WDW and asking them if they would transfer the reservations to different days, possibly. This just sounds too risky to chance. Not only for the child who might become ill while on vacation, but for all those who may also be affected.