I have a 9 year old daughter who has multiple disabilities including profound apraxia. We have been told by her therapy team that realistically she will more in likely never be able to use speech to communicate beyond a handful of words most of which are very segmented. She uses a combination of sign language and proloque2go on the ipad with a dynavox maestro and PECS Cards that have words written on them to go with the pictures as a back up system. We take at least one back up to the ipad when we travel.
So far we have only been able to make 1 trip to WDW when she was 27.5 months old. The only character she was really interested in and wanted an autograph from was Minnie Mouse ( we got other ones for her). She was able to let Minnie know by holding out her autograph book and signing Please.
OP if you can I would recommend purchasing an ipad that your family will own for her and purchasing the Proloque2go. Even though you won't use it at the time get the bundle that includes prologue4text along with 3 other apps 2 which teach keyboarding ( one is designed for elementary to middle school the other for middle school to high school) because the money you will save over purchasing them individually in the future is worth it. Once you have them purchased they stay on your cloud. We have pulled up prologue2go on a my 11 year old daughter's ipad though it was a bit slow due to being an older version and less storage space overall. You will also then own both the device and the app so you will have more control as to what is put on it so if you want to put words used in a religious setting for what ever reason say your going to a coming of age ceremony for someone of a certain religion and you want her to be able to congratulate the person on the accomplishment if you own it opposed to the school district owning it ( when you say preschool I am making the assumption that it is a preschool program ran under part B services for children between the ages of 3 to school age that either have a documented special needs or are at risk for a disability based on the laws of your state) and even having to go through this again when she ages out of those services. By owning the ipad you will also be able to have other apps as needed such as reading rainbow ( while you can't change any of the 5 books in the backpack when your not connected to the internet you can read the ones that are in there or using earphones ( in public) to have it read to you as you follow along) which can keep a child entertained during travel.