When my brother got sick I made him hospital gowns to match the seasons- used 100 % cotton. They wash up well and he got a kick out of the Halloween ones
Maybe find fabrics with her hobbies and interests or that suite her personality. Sadly he never got to wear the Christmas ones- I used the fabric to make ornaments for us
When looking at fabric- how it feels to your hand is a good indicator on how it will wash up - so choose as soft as you can find it - also don't worry too much about the back ties - we found they got in the way and were uncomfortable when he laid on them - I just made sure the back really wrapped around