I have been to WDW when my girls were that age(we always had a great time and it was VERY easy), but never in the hot summer months. I would ask you pediatrician about sun block, or what they would recommend to use. MAke sure you have a nice big hat.
There are several companies that make Solarveil clothing or blankets (the Solarveil has sun protection) It is a VERY light and breathable fabric. You can also get a solarveil or solarweave(much softer) slings if you are interested (you can thake it right in the pool too..lol) Hotslings.com sell some and chicslings.com have some.
I would be VERY careful about using an infant seat.they get VERY hot inside there, if you do make sure to line the inside with a white towel..or if using a regular stroller do the same...
I would say make sure to nurse often (if you are nursing) or make sure the baby is getting enough fliuds from formula.
A clip on fan for the stroller is good, and mae sure you get in the a/c often.