Welcome
My youngest DD was diagnosed with cerebral palsy (CP - so you don't have to type it all out) at 19 months. She is now 24 (time flies!).
Our first trip to WDW was before she had a diagnosis, but it was when I really noticed there were some things 'off' about her. She was very tiny and had done pretty much everything on schedule developmentally (except walking), but just never did it well. When we were at WDW, we rented one of the park rental strollers one day and I saw that she had a tremendously difficult time keeping herself in an upright sitting position. She kept tipping over.
It took until just over 18 months for me to get the doctor to listen, then she was referred to a very nice neurologist who observed her for a bout a minute before coming up with a diagnosis.
We've made at least one trip a year since then (the last 10 years, 2 trips per year).
My advice for the parent of a very small child with CP at WDW is that there are many things you can do. Many attractions are safe for babies, so even if her head control is not good, she can be held on the ride until she gets too big for holding on your lap. Any attraction without a height restriction is probably going to be safe. My DD has some perceptual and attention problems also, which made some of the very stimulating rides like It's a Small World kind of overwhelming for her. I have vivid memories of her on Small World somehow covering both her ears and eyes with her arms, while still managing to suck her thumb and twirl her hair for comfort. SW was not one of her favorites for a long time, but she likes it now.
If you have any specific questions about WDW or about CP in general, or care of a child with a disability, you have come to a good place.
He has other issues such as vision problems and sensory sensitivity. Small world. 
