I have no idea as a photographer but as a parent of a disabled child I will stick my 2 penn'orth in...
For me, I know my child has a disability, I know she can look at you in a funny way, with a squint, and with her glasses and her hearing aids. I know that she stands funny with her weak muscles and her walking is like a 1 year old even though she is 3.
What I would want a photographer to do is to show my child just as she is...don't try to hide what is there...the parents know their child and will expect photos of their child, not someone elses perception of the child. I know of people who have taken photos of children in a wheelchair and then cloned out parts of the chair. But the parents expected to see the chair...
...but, there will always be some parents of kids with disabilities that do want stuff changing. If there is any way to speak to the parents beforehand, then do. And don't be embarrassed by asking...most of these parents do not have the embarrassment gene in their bodies anymore...I know I don't.
Hope that helps.