There are a lot of differing levels and types of CP. It sounds like this girl has a milder version than many people.
Maybe ask for a brief summary of abilities from the student's doctor and/or physical therapist? They might also be able to identify what the student should work on (balance, communication, etc) and any restrictions she may have.
If she has mild CP and is able to follow your directions in the case of an emergency then teach her as you would teach another five year old rider. Do you teach children that young? A report from her teacher or therapists might be helpful to help see what her level of understanding is- usually there are no cognitive complications with CP. If you do get reports from her teacher or therapists, remember that your goal and job is still to teach riding.
Ask her parents if she's had any surgeries that might impact flexibility/function. While I have a different neuromuscular disorder than CP, I had surgery for a club foot which makes the concept of "heels down" virtually impossible for me on one leg since that ankle and tendon/heel cord are messed up. My instructor really worked on my leg to establish tightness and security even without a lot of weight in the heel. Now I'm secure and stable enough to wear spurs, etc.