I know we have other CP families here that will be along, and they may have some actual experience at WDW with this.
I am guessing that you are talking about at the Resort hotel pool, and not the water Parks, correct?
You may have to explain to each lifeguard during your stay *why* you need to go down the slide with her (in other words, kind of like when you talk to Guest Relations at the Parks about how a
DAS can help your visit) and so when you have that conversation, don't focus on *what* her diagnosis (CP) is, but *why* you need to go down the slide and in the water with her (because she does not have the ability to sit independently at this time)
Does all that make sense?

When you have a kid with a permanent health issue like CP, we sometimes forget that in our world,*everyone* knows that if you have X, then Y.
An example of that is my daughter - who was born with bilateral club feet, and multiple other orthopedic issues. We were so immersed in her care, and I dealt so much with doctors and physical therapists, and they all *knew* that she would never have walked without extensive surgeries and treatment and PT and braces and special customized shoes and... So when she started school, I suddenly had to explain to a lot of people who had never dealt with a kid like her before what was going on with her physically, why she had a wheelchair, why she would never have perfect attendance (because the Shriner's Hospital was an 8 hour drive one-way from our house) and when to call me.
We get so immersed that sometimes we forget that people can't know what they don't know
And remember - it may be that they can't let adults do that for some reason; like there is a weight restriction on the slide, or because of some other water safety rule that they may have to obey. It may not be that the CM lifeguard wants to be the bad guy; that decision may have been made for them.